Thought I'd jump on the Game Boy bandwagon. Pretty self-explanatory. What games and features do you most want to see on the service? (I'm assuming the GBA isn't part of the equation, and I have almost every GBA game I'd want, anyway, so whateverrrr.)
As with the VC, I'd love to see all of the weird rare gems that I never got to play (going handheld in the GBA-era): Shantae, Kid Dracula, Gargoyle's Quest, Mole Mania, Balloon Kid, Wave Race, whatever Kirby games weren't ported, any original entries in beloved franchises, in general, some Japanese stuff that never came over (like Aonuma's adventure game whose engine was the basis for Link's Awakening), and also all of the Mario Land games, since I've never played them. (I wonder if they'll put older Pokemons on the service.) I'd also kill for Virtual Boy support, since my Virtual Boy broke and I didn't have many games, anyway. Don't really care about the other handhelds of that era. Lynx was kind of cool...
I'm really not sure if handheld games of that era will hold up, but I'm probably biased because the only GB games I played in my youth were, like, licensed shit from LJN.
Also, I hope Super Game Boy is supported, unlike on the GBA. And wacky 3D, as well. What the hell.
I hope the old Kirby games show up. I never played them, so it'd be cool to be able to for cheap. Considering the fact that's it's Nintendo's own IP, I'm pretty sure it'll happen.
Motocross Maniacs was another game that I loved as a kid and would really like to be able to play again. It was like a strictly 2D Excitebike-type game from Konami, but with Sonic The Hedgehog-like loop-de-loops. It was pretty awesome.
The original WaveRace would be a hoot to download. I actually still have the cart, but... I'd download it just for shits and giggles.
Other than that, I'm of course game for all the standard-issue 3rd party stuff; Mega Man, Castlevania, Contra, Final Fantasy... stuff like that.
As for other handhelds to be supported... I'd really, really, REALLY like it if the Neo Geo Pocket Color was supported. The system was short-lived, but man did it have some awesome games.
I want to see these old games updated. For example, online for the Pokemon games like Secret mentioned, and like stephen mentioned, wireless functionality. Maybe even some leaderboards too for some games.
Link cable and IR emulation would be great, but probably pretty tricky.
@X-pert74 They started releasing Game & Watches on DSiWare, but I don't think anyone cared. I'd rather have spruced-up collections (that still allowed you to play the originals), like the G&W Collections. How the DS, a DUAL-SCREENED system, is missing a proper Game & Watch Collection is beyond me.
I'd like to see GBA games in there. I don't know that much about handheld games prior to that, except for the big names like Super Mario land, Zeldas, etc. Nowadays it's pretty hard to get some of its games (for the GBA, I mean) and there are a lot of games I know that I would like to try out. With that being said, I know there's no guarantee that they will com to the library.