I'd steal a few ideas a friend of mine had.
I'd publish a book of themes, or in his words, "lenses" - ways in which you can tweak the rules, alter the way you write up adventures, and other techniques for imparting a certain kind of feeling to the game. It would be very much like a collection of the "Campaign Components" articles from Dragon Magazine in the last few years; I was just reading through the Swashbucklers article today.
The book would combine player-focused material like feats, equipment, prestige classes et cetera, with DM-focused content like advice on maintaining theme, how to structure genre-appropriate adventures and story arcs, and so on. Each chapter of the book would cover a different theme or genre - horror, swashbuckling, political intrigue, et cetera. The last chapter in the book would contain DM-focused advice on combining any of these genres with the standard "heroic adventure" theme of core D&D, as well as with each other. The type of content in each chapter would vary as necessary - the horror chapter might have genre-reinforcing mechanics like Ravenloft's fear/horror/madness checks or sanity, while the political intrigue chapter would be focused on subsystems for playing PCs who are political figures to be reckoned with, and for running games featuring them.
I would probably expand the "adaptation" advice that's now being given for how to alter or develop prestige classes to fit your campaign. I'd have my designers go into more detail on several different kinds of organisations or traditions which might teach the skills of the class, and give advice on how the standard races of the core D&D world, at least, would treat the class. They really need more flavour and flavourful ideas associated with them.
However, despite my affection for prestige classes, I would urge my designers to explore other options. Feat chains, for example, are often better-suited to developing a certain specialised concept than prestige classes; I would focus quite a bit of energy on creating feats and feat chains aimed at high-level characters.
It has also come to my attention from reading older Dragon issues that some interesting Third Edition mechanical concepts like martial arts styles have fallen out of favour with designers; I would not only encourage their use but also expand them to other areas, with particular attention to developing more skill-related feat trees and associated "mastery bonuses" as a way of deepening the use of the skill system. I would also look into spellcasting-related "styles".
I would also, perhaps, take the space to add a little bit more flavour text for each feat, along the lines of the various Dragon articles about what characters' feat choices say about their personalities. It would be a design priority across all lines to increase flavour, even if only by adding little hooks here and there that players stuck for cool ideas can sieze and elaborate upon.
I would, if I was honest with myself, do something to please the fans of older campaign settings. A slim, Book of Exalted Deeds-sized hardback campaign setting for Greyhawk, Planescape, Spelljammer, Al-Qadim, et cetera, would be cool. I would probably sacrifice the high production values of the main line, however - black-and-white interiors, for example, but make up for it with good writing that hearkens back to the roots of the setting. I would, perhaps, even contract the books to freelancers associated with the original settings. Either that, or sell the rights for a single product to a third-party publisher; I'm sure all of these settings could find a home somewhere, with contracts which keep the IP safely in my hands.
(I'm assuming here that I'm running a Wizards that's still owned by Hasbro, and thus have to "play by the rules" and not just sell off Greyhawk and the like, or cater in expensive fashion to tiny market segments.)
I would look seriously at selling niche-market products via PDF, since it seems like smaller companies are doing pretty well with it, and it's a lot less expensive than doing print products.
I would then come up with the most popular setting ever created, and make millions off it.
)
D), I'd then publish books detailing the various aspects of the world - similar to the Gazetteer format many companies are using, but not only encompassing geography and history, but also options for players and DMs alike to create characters from that region, giving detailed background information about culture, customs, rites, religions (if any), etc. Another book would contain the setting's monstrous inhabitants, and would feature a style similar to the Iron Kingdoms book - there's nothing that beats a long, flavory description of a monster that you can build on in describing it to your players. Yet another book would detail the major organisations, their aims and plans, their history, give example characters, feats and/ or skills and spells specific to these organisations, (but not just because the back cover needs to read "100 new feats!")which fit the flavor and mood of the setting.
.
