Modularity: a core with the simplicity of TSR-D&D, and targeted expansions for those who want the complexity of WotC-D&D in one area or another.
For example, there would be provisions both for the old resource management approach and for the current "super-heroic" mode. Combat could be quick, or "tactically detailed" (with grid and so on) depending on each group's preference. Likewise, a "skills system" could be used or not. Feats and powers, if desired, would "swap out" regular class features, with a system of "slots" by level (a la 4E) -- but class selection itself could again be "one-stop shopping" for those who like a truly class-oriented game.
Classes balanced by level: This seems the preferred approach of most players, and 4E does a good job of it. However, I would back off from the tight focus on combat power and return to "spotlight balance" with wizards and warriors having clearly different spheres. The "heavy artillery" spells so entrenched in D&D tradition would remain, but be subjected to revision as seemed meet.
Well-considered power curve: Trying to maintain the same probabilities to hit while boosting hit points creates problems to my mind, and similar problems arise in other areas. The trouble with an open-ended "more of the same" approach is not much ameliorated with a 20th- or 30th-level cap. Too-rapid level advancement gives too little time for players to get acquainted with character abilities before taking on a new set, rushes "story" development, and makes too little use of level-rated elements such as monsters (or leads to warping them beyond recognition, thereby reducing the ability of players to gauge challenges). It also feeds into the factors that make "magic item upgrades" a pressing concern in some quarters.
Bring back the magic: The reduction of many magical effects to minor quantitative factors little resembling what "names to conjure with" conjure in the imagination seems to me a disservice to fantasy fans.
Return of henchmen to the fore: Primarily, they keep a player from being removed from play when one character is inactive -- a fundamental problem with characters that are not invulnerable. Their strategic and relationship aspects also enrich the game, and keep charisma from being a "dump stat".
Broadened horizons in scenario design: The tight focus on strings of "encounters" (as redefined) is a fine approach, but it has shoved all else almost off the scene. I dig the classic tournament modules, but there's a lot more to D&D. I would give different approaches their due. In particular, I would not build in "encounter XP" as the default method.
Informative and unique keywords: "Fireball" is pretty clear. Without looking them up, who would have the foggiest about many powers (especially the "martial exploits")? And who thought it was a good idea to have the Cyclops Evil Eye mean any one of half a dozen completely different things?
Return to roots: There's a place for someone's proprietary fantasy, but I don't think that place is dominating the core of D&D. Case in point: the deities in the 3E and 4E PHBs. Greyhawk, the Forgotten Realms, Planescape and so on are nifty -- but they lack the wide recognition and deep resonance of real myth cycles. Turning to the old Deities and Demigods (later Legends and Lore), one finds an array of mythologies including those from which most of the classic monsters were drawn. Likewise, there are character class archetypes and "races" not only traditional in D&D but easily recognized by newcomers.
No "power creep": More peculiar creations can provide plenty of material for supplements, but they should not make core material obsolete.
Tools for adventure: Ongoing games provide a continual market for material of use to playing groups. I see the rationale behind trying to sell ever more rule books to character players (as opposed merely to DMs), but I think one might accomplish much the same with products aimed more directly at enriching actual play of the existing game. That might mean simply offering more and improved "modules". It might encompass separate packages for players and DMs. Most of all, I think it means delivering material so widely useful, inspiring, carefully designed and enjoyable that frequent purchases become par for the course.
Easy entry into a perennial and expanding line: After the dedicated fans have bought the first volumes of the latest "new edition" which is really a new game, visions of sugarplums may dance in the heads of those who anticipate selling all over again the same batch of stuff as in the last round, only "updated".
One problem, though, is all the investment needed to rewrite so much to bring it in line with the new game. Another is that some people will decide that they don't want to go through replacing their old books -- and have little use even for really new offerings, because they're for a different game. A third is that newcomers to the hobby are greeted right away with the prospect of a big investment that may be "outmoded" before they are much of the way through. Not only players but retailers can get stuck between a rock and a hard place when a new edition is announced.
Above all, I suspect that this turns off a big potential market.
An inexpensive essential set that's easy to get into can continue to sell for decades as new players are introduced to the game. Those new players will buy old supplements as well as new. Once design and development costs have been recovered, unit sales are "gravy". Repeatedly putting everything to ride on a whole new product-line launch can be a great success -- until it's a disastrous flop. Hollywood, I think, can handle more flops than an RPG publisher.
Of course, Hasbro probably sees things differently. D&D is just a minuscule part of its diverse portfolio. It could be dropped for more of My Little Pony, or whatever else might offer a better return on investment at the moment.
