It would depend on whether my mandate is "create the game that's best for me" or "make money".
First up, whatever I'm doing, 4e is gone. Not because I think it's a bad game (I don't, although it's not my preferred edition), but because there are a number of fundamental issues I have with the game that I don't see how to fix without a new edition. (FWIW: the constant rolling revisions; the explosion of options that has made the Character Builder an essential tool for me to play the game, where I object to any required electronic component; and the combat grind are my absolute deal breakers.)
So, I'd start the team working on a new edition. My 'ideal edition' would look a lot more like 3e than 4e, but would actually look more like SWSE than either. Even then, though, the resemblance isn't all that close, as there have been a lot of really good ideas since then that should be incorporated. But certainly, it would be a lot more rules light than 3e, and a lot more option light than 4e, at least (and especially) for new players.
Incidentally, there would be four tiers, as in BECM D&D, running from levels 1-3, 4-14, 15-25 and 26-36. I don't want to go into details (they're very woolly even in my head), but that detail becomes kinda important later.
Setting support would resemble that of 3e-Eberron or 2nd-Ed-Al-Qadim; that is, a reasonably small set of books that covers an area in some detail but not too much. This is similar to the 4e model, except that I feel they've gone too far in having only three books.
As far as adventures are concerned: the Delve format is (mostly) gone. It was a noble experiment, but it seems to stifle writers. (Of course, WotC have never really excelled at adventures regardless of format - they have produced maybe half a dozen good adventures, ever.) Where it does work really well, though, is for zero-prep adventures for the time-pressed DM. So, I would keep it for "Dungeon Delve" type products.
But I would definitely look to signing up some third-party support, at least for adventures. I suspect Paizo wouldn't be interested in supporting any new edition, but maybe Goodman, ENPublishing, or some startups? (Alternatively, just bring those third-party designers in-house, and have them take over adventure writing.)
Unfortunately, I think there's no scope for bringing back the paper mags either, so that would have to continue on an electronic basis. Shame.
Now, if I'm just making the best game for me, my ideal edition would be published as six books (rule book, classes and feats, the grimoire, tome of treasures, and two big books of monsters). And that's it for rules material for two years.
After two years, the design team would do an "errata cycle writ large" to produce a new, better version of the same game. The scope of the changes here is definitely less than "3e -> 4e", or even "3.0 -> 3.5". It really is just a matter of assimilating those changes that two years of play have shown to be necessary. Republish the same six books, and then close up shop, as far as rules are concerned.
For settings, I would support only one: Eberron. If Paizo could be persuaded to turn Golarion into an official D&D setting for my edition, that would be fantastic also.
Other support would take the form of published adventures, skewing heavily towards the supported setting(s), plus monthly eDragon and eDungeon.
And that's it. We carry on until it's no longer viable to do so, then wrap up the support.
Of course, I'm deliberately setting a course towards destruction for the game. Without any means of drawing in new players, without any DDI drawing large numbers of (player) subscriptions, without any notion of Organised Play, and without any consideration of licensing, the game will founder and die. In short, it wouldn't make money.
If my goal is to make money then I'd take a different tack.
Here, my priorities are as follows:
1. Attract new players in large numbers (relatively speaking).
2. Keep those players spending money on our products.
3. Where players do leave, try to keep them in the "gamer family" in the hope they'll eventually come back. Meanwhile, keep gaming in their consciousness through carefully targeted ads and product placement.
I would position D&D as "a boardgame without the board", rather than as "an MMO without the PC". What WoW does, it does better than D&D ever could, so competing with it on its own terms is madness. D&D needs to operate based on what it does best: getting people together in the same room to play, cooperative storytelling, and ongoing narratives.
So, my new edition of D&D would be released as two products on day 1. The main product (called simply D&D) would consist of a great big box including everything you need to play. So, we'd have a Quick-Start guide, the Core Rulebook, a DM's book of sample adventures, dungeon tiles (to support the initial adventures - the game wouldn't require the use of minis by default; but see later), appropriate minis, dice (preferably several sets), pencils. Heck, even a CD including a character generator, character sheets and forms, printable handouts, more adventures, and a link to our online support.
The second product, aimed at existing players, would be the same Core Rulebook, sold separately. This would include details of the rules for the Beginner and Expert tiers, a limited set of races (Human, Elf, Dwarf, Halfling), a limited set of classes (Fighter, Rogue, Wizard, Cleric), a smallish set of spells, monsters and treasures, and so on. One rulebook, of no more than 250 pages. If the game doesn't fit a book of that size, it's too complex.
Did I say two products? Well, I lied. I'd also sell the dice and dungeon tiles separately. I'd try very hard to just give away character sheets, certainly online, but in print if possible. The other day 1 product would be a "Dungeon Delve" style book of adventures intended to keep people playing, and designed to run with no prep. (See my comments on the Delve format, above.)
Thereafter, I would gradually release my "second circle" of rules products, designed to provide the missing bits from the Core Rulebook. So, here's where you find the Gnome, the Druid, levels 15 - 36, the missing dragons, and giants, and so on. There would be four such books, being Character Options, the Grimoire, Tome of Treasures, and the Expanded Monster Manual. The "second circle" would be designed assuming you're using all of them together, combining with the Core Rulebook to provide the 'complete' game.
Importantly, the game would be set up assuming you aren't using minis in the first instance (while still allowing for their use). I would later produce a "Miniatures Handbook" providing the detailed rules for using minis in your game. I would make it very clear that this was an optional component of the game... but I would also make damn certain that the game played better with minis than without. Cos I'm evil.
And, adding to my evil-ness, the line of official D&D minis would have been allowed to go out-of-print for that period when I'm developing my new edition, and then relaunched. Of course, the new minis would be designed at a slightly different scale, or otherwise to be just different enough to not quite be compatible.
I would aim to produce one setting every two years, as discussed above. The first setting would be Eberron, released some 3 months after Day 1. Then, probably, Dark Sun, or maybe Planescape, or even Spelljammer. The Forgotten Realms I'd keep out of print for at least 5 years (10 would be better), including the novel line and any licensed products. Then I would "reboot" the setting with much fanfare. The timeline would be comparable to pre-Spellplague, but the designers would absolutely not be beholden to what went before.
(Oh, and produce a new setting aimed directly at adults. Not gross and edgy 'maturity' as in "Book of Vile Darkness", but something with some real meat and, dare I say, literary quality to appeal to adults, rather than teens trying to rebel.)
Finally, there would be ongoing support with lots of adventures. Some would be designed assuming just the use of the Core Rulebook, and some for use with the "second circle" products. (Obviously, these would need to be clearly marked.) At least once per year, I would also be sure to put out another "Dungeon Delves" type product of zero-prep short adventures for use with just the Core Rulebook. Adventures would gradually drop out of print, and would be removed from sale even online.
Online, I would be looking to keep the DDI working mostly as-is. The ongoing revisions would be gone, of course, although we would issue one set of required errata for each book as needed. But the range of tools is mostly fine as-is. The online mags would need beefed up, though, and the quality would need to improve. eDungeon should offer 5-6 short adventures each month (open this to submissions from outsiders; these should be regarded as the primary means for people to break into the industry), plus an ongoing Adventure Path (roughly 1 AP per year).
The online tools would also be the only place to get some forms of support. I would never release a Power Cards product in-print, for example, but provide this only online (the cards would be customised to the particular character, as are the Character Builder's cards, and so not easily transferrable). And, as with the minis, while I would make it clear the online component was an option, I would make damn sure that the game played better with it than without.
The DDI would also be the releasing ground for all new character options for the game (excluding setting-specific stuff). And, again because I'm evil, the online options would probably be ever-so-slightly better than the in-print stuff, and gradually creep in power over the edition cycle.
(The DDI would include a virtual tabletop, but this should be considered a low-priority item. We want gamers to be in the same room to play; playing online should be supported for those who can't get together, but isn't our priority. Mostly because people who have to game exclusively online would probably be better off with an MMO.)
I would make sure to research the "gamer half-life", that is, the period of time in which the pool of gamers churns by about 50%. This would become the lifetime of an edition, repeating the pattern, but just incompatible enough to require replacement. (Obviously, make sure the new edition is somehow better than the old.)
About halfway through the edition cycle, I would remove the existing minis from sale, and ramp down production of Dungeon Tiles (in preparation for a slightly-incompatible relaunch with the next edition).
At the same time, I'd launch a new "D&D Starter Set", covering the Beginner tier with a very cut down set of options (but the same underlying rules - not a dumbed down ruleset). Crucially, it would include rules for real character generation - WotC still haven't grasped that that is an absolutely essential component of a starter set (IMO).
This would be followed by a "D&D Expert Set" expansion, covering the remaining ground from the Core Rulebook, but providing a gentle entry route to a now-bloating game.
WotC are actually doing a really good job with Organised Play (IMO), so I'd just carry on as they are doing. I especially like the little rewards given to people who are involved ("The Tomb of Horrors" conversion, for example, was inspired).
Now, licensing. Try to get the rights to a D&D movie back. Anything short of the absurd should be done here - we need a new D&D movie, but it must be done right. The existing semi-serious abominations should be wiped from existence 1984-style.
At the same time, I think there's scope for an Eberron movie. Again, it must be done right - take it seriously like "Lord of the Rings", not the standard "geeks are idiots; they'll watch anything" approach to fantasy.
Likewise, I would strongly look for licensing options for new D&D games, both bearing the generic D&D tag, and also for Eberron and other settings (but not FR until after the reboot).
When the time comes for the FR reboot, I would try really hard to have it coincide with a new computer game (NWN 4?), a new novel line and, if I can somehow manage it, a new live-action TV series.
Oh, and a new D&D cartoon, and a continuation of the D&D comic book lines, would be great, too.
Finally, push for D&D product placements, and if the budget allows, even go for actual TV adverts. Again, focus on "group of friends in the same room having fun", rather than "lonely geeks who can't get a date". Here, edgy is better than geeky - people will buy a game that pisses off their parents; they won't buy a game that suggests they'll never get a date.