As I was saying before, I would like it if D&D had official support within the D&D brand and ruleset itself for kinds of fantasy other than pure traditional medieval fantasy.
For example, D&D could have some options that would support something like the
Star Ocean series of videogames. These games are still all about characters running around fantasy settings killing monsters and taking their stuff, with the one difference that some of the characters happen to be have beamed down to the planet on an away mission for an organization similar to
Star Trek's Starfleet (a version of Starfleet where you can major in swordplay or sorcery). So, it would play a lot like traditional D&D, except characters might wander between planets, use beam weapons, or get caught up in a space dogfight. Comparisons to the
Phantasy Star series of videogames might also be appropriate.
Alternatively, a few options to emulate something like the
Wild ARMS series of videogames would be nice. Again, the
Wild ARMS games play a lot like normal D&D, except the characters all use firearms, there is a slight wild-west flavor, half the dungeons they go into are ancient spaceships, and demons know how to control nanotechnology. Actually, a great DM I used to play with told me he once ran a campaign that was very strongly inspired by the
Wild ARMS series. It would be nice if the game itself did a better job of supporting that.
Actually, the whole idea that a fantasy setting is in fact the product of some kind of post-apocalyptic future Earth is pretty common in some of the major source material for D&D. Vance's
Dying Earth and Terry Brooks'
Shannara books both feature that idea, as well as several others I could name. I mean, I quite clearly recall a scene in
The Sword of Shannara where the main characters are attack by a giant robot that was left over from the wars that destroyed the older civilization of Earth. The idea of "lost technology" that can be found and used is a pretty major idea across countless anime and videogames as well...
Generally, a few options within a PHB or DMG for guns, beam swords, cars, spaceships, robotic monsters, and various other trappings of modern and futuristic settings is all I really want.
Also, do you see how people might consider it sufficient for D&D to do "its own thing", just as other games do other things -- including some (e.g., Shadowrun) that do Cyborg Orcs With Big Guns, Magic Spells And Mocha Lattes?
I don't think there is really a contradiction between D&D "doing its own thing" and letting it branch out into a few other genres of fantasy.
After all, as several other people (including you yourself, I believe) have quite strongly pointed out, the idea that such things can be found in D&D has dated back to the 1E days where you saw things like "Expedition to the Barrier Peaks". Why shouldn't there be more direct support for that kind of thing, then?
At the same time, D20 Modern and Alternity created numerous interesting and unique IPs related to modern and futuristic settings that, for some, are as memorable as anything seen in D&D proper. However, I don't really see the need to keep the separate from the ideas of D&D, when they could work side by side in the same ruleset. The fact that D20 Modern allowed D&D-style fantasy, a modern setting, and Alternity-inspired futuristic stuff to coexist shows that it isn't impossible.
Also, since you mentioned it, I want to say that Shadowrun is
not what I am looking for. If nothing else, Shadowrun is a case where the setting and the game are inseparable, and I have no interest in the particular brand of distopian modern-future fantasy mix-up that Shadowrun offers. I want something modular and flexible, that lets me create my own setting the way I want. What is more, Shadowrun is not D&D, which is both the biggest game on the block by an order of magnitude or two and the only tabletop RPG ruleset that I have ever cared to learn (well, 4E, 3E, 3E variants, and a little 2E and Alternity, so I guess a singular usage isn't quite appropriate...).
Still, I guess it is worth mentioning again that this whole discussion started out as my response to someone saying that D&D could be seen by people of my generation as the embodiment of all the fantasy videogames and such that we grew up playing. Simply because I can so easily name countless fantasy games which include guns, robots, spaceships, or whatever, that statement was false. I think people keep assuming that I am saying that D&D is somehow unplayable or unpalatable to me because it lacks those things, but I haven't said anything like that at all. I mean, it would be
nice if I didn't have to create stats for guns and such when it was necessary, and it would be really nice if I had a better baseline to work with in designing my own D&D variant based around Gundam-style mecha (and that it wasn't so
hard coming up with 4E-style classes thanks to all those accursed powers!), but I have never said that D&D is a bad game because of their absence. After all, I really enjoy D&D, and even though 4E is the only edition to be completely lacking modern or futuristic stuff so far, it is still my favorite edition.