Pure and beautiful integration.
I want, in a simulationist style, a world (or worlds) including planes, history, future, npcs, adventures (good ones) , etc.
I want them all to fit together, if not seemlessly, to fit well.
Open design is doing this right now with Midgard. WotC (and hell, 3pps) used to do this in 3e well enough. But I'm not a "system-itist". Midgard might be 4e, pathfinder, another system, or systemless. I'm in for over $100 regardless of which system is chosen in voting, because they ARE doing this.
Here's a project I want to personally undertake in my retirement (when batteries work for 100 hours and laptops are beach and mai-tai friendly):
Integrate it all. All of it.
I have lots and lots of books. I'd love to create "hell" (for instance) using all the main books along with all the third party supplements, rules, etc. I'd like to construct a "sandbox hell" including all the adventures that relate to the hells.
That's just my first step. I'd love to do it all.
Multiple worlds, multiple planes. My ideal dream is the "ubersandbox" of "every book I own fits somewhere and and somehow".
FURTHER POINT: I feel as though most of D&D fit this model up through ogl/3e, including many, many third party publishers who were riding official D&D fluff, or could easily fit with it at the least. I feel as though there was a huge departure by WotC with 4e from this fluff (intentionally, but against my wishes --who knows, maybe for the best?). WotC reinvented much of the fluff/story/worlds and so such material breaks from my goals. What I'd need to work their current material into my vision is future material that reconciles weird changes in a thematic/realistic(ish) way. E.G. the feywild sprung up from the plane of shadow when....x, y, or z...and that fits into previous conceptions and incarnations of the world in a number of the following ways.
Part of what excites me about D&D is the "reality" of a fantasy world. Fantasy itself is fun, but only so fun for me. I don't really enjoy reading fantasy books, because they often seem contrived. The best ones don't, and, IMO, the reason they don't is because they create vibrant worlds.
I like D&D because, when done well, it doesn't seem contrived. That's what I want as my WotC fantasy product (both meanings of fantasy here): for them to create a playground that is different from reality, but can seem just as real.