First things first:
avin said:
"The World" idea would be burned as a witch on a bad night,
Excuse me? Witches don't burn anymore, thanks.
Now, on to 5e...
First off, if I were alone at the helm I'd be designing for enjoyment first and profit a far distant second, with any profit that did happen by considered as a bonus. (in other words, for-profit publishers can stop reading now; my ideas are not going to help you very much)
The game would be a double-barrelled setup, with an introductory/basic style game that is fully playable as is (and is designed for shorter campaigns, tournaments, one-offs, etc.), and an advanced/elaborate game (designed mostly for long campaigns) for those as wants it. All elements would be fully compatible one to the other, as far as possible, and where possible also backwards-compatible to one or more earlier editions of the game so those who wish to continue playing such can still gain something useful from 5e.
There would be several "core" books (in a boxed set?) to begin with. For the intro game:
- a Players' Book, kept as cheap as possible but containing all rules and info required to roll up (yes, *roll* up) and play a character; written clearly but flavourfully - think the 1e PH in style only more organized and easier to navigate.
- a DMG, off limits to players, containing all the information needed to run a game including elements from the 1e DMG (charts, tables, etc.) and the 4e DMG (the 'how-to' stuff). It'd have write-ups on different types of games and how best to run or combine them.
- a monster guide - we all know how these work
- a world guide. This is new, and contains all the information required to build a living breathing world including deities, cultures, etc. A DM could use the example world included, or use the info to build her own; whatever. But get the idea in from the start that there's more out there than just the party, a tavern, and a dungeon.

Again off limits to players; and while a game could be run without it, having all this in its own book leaves more room in the DMG for other things.
For the advanced game:
- an expanded Players' Book, containing a short form of what's in the basic PB along with whatever enhancements are involved in the advanced game (you'd get this *or* the basic PB, depending what you wanted to play)
- an expanded DMG, ditto.
The monster and world guides would work equally well for both.
The base game in both variants would be kept simple for the player. No feats, no powers, no memorized spells, etc. For the DM, however, running things gets a bit more complex - not in terms of prep, but at the table. The player rolls the dice, the DM figures out the result - much like 1-2e. About 10 core races. About 15 core classes with clearly-stated limits on multiclassing (i.e. if you want to multiclass, fine, but know going in you'll never be as effective as a single-class character). And stop there. That's it. No more.
Expansions and splats:
- Optional rules expansions - one for each previous edition, to make the new 5e system at least somewhat compatible with same. (the 3e guide, for example, would put in things like feats, skills, etc.; the 4e could include move effects, dailies, etc.)
- Errata are published online as they are found and are rolling-changed in the print books as they are produced
- Every now and then, a new monster guide incorporating ideas thought of since the last one
- Adventure modules. An ongoing stream of these.
- Settings. An ongoing stream of these too, but incorporating as few setting-specific rules as possible such that a party from one setting can easily jump into another without a bunch of mechanical changes being required. Each setting gets one book or boxed set and that's it.
- NO additional class splats, power splats, etc.
As for mechanics, while I'm not going to redesign the game here and now, some things I'd sure like to see:
- More use of d%, and of all the other dice in the bag.
- Commoner-level character design for those as wants it - the gap between 0th and 1st level should be less than the gap between 1st and 2nd.
- Character generation made very simple - in the basic game, an experienced player should be able to crack one out in well under half an hour.
- Scaling - low-level characters should be somewhat ordinary, and aspire to become high-level characters who really are powerful - like 1e but not as extreme. Levelling is slower than 3e/4e but is open-ended; the game would not cap out or break at a certain level. The tables go to a certain point but can be mathematically extrapolated from there.
- Where possible, rules would at least try to reflect reality. Re-rolling initiative each round is one easy example, to reflect the confusion and lack of choreography in a melee. Falling damage is another.
- More emphasis on social standing and interaction with the greater world.
- "There's always a bigger fish" - the PCs know they are not the only adventurers or levelled people out there.
- Let spells do their jobs, and let players be creative with them; in other words, de-nerf them If something breaks badly, that's what errata are for.
- Scale back armour and protection effects somewhat - make to-hit probabilities a bit better to avoid combat grind.
- Make the 3-18 ability scale mean something. A 19 is rare. A 20 is almost unheard of, even among the most heroic of adventurers. (this also gives room to scale up monsters, who of course have no such restrictions...a Dragon *can* be Strength 27 and get all the commensurate bonuses)
- Make mini use optional by streamlining combat rules.
I could go on and on and on, but you've all suffered enough for one day.
Lanefan