I'd love to play DS games on the TV - is there a way to show both screens on both the TV and the DS?
My friends and I love playing through Super Mario RPG, and I keep telling them we should find a way to play through the Mario & Luigi games on the TV, since they are the closest to a follow-up we'll get. (Paper Mario is fine, but it's different and unique enough that I don't consider it any kind of Mario RPG sequel) Obviously we'll play Superstar Saga via the Gameboy Player, but it'd be nice to also play through the other 2 games together.
For Game Gear on the TV, it would be cool if some place like Retrozone could create a sort of Super Game Boy for Genesis - they already have a Super Game Boy for NES. Something for those of us who don't really open up our cartridges and start messing with stuff.

EDIT: Looks like this was explored here, by way of Game Gear to Master System, and Master System to Genesis with the Power Base Converter:
http://www.sega-16.com/forum/showthread.php?10539-Game-Gear-to-Mega-Drive-Genesis&highlight=game+gearAnother thing I would like to be created is a console similar to Retron3 (even though I hate that system). But for Atari 2600, 5200, 7800, Intellivision, Colecovision. These systems are getting harder and harder to come by and I think a new clone system like this could be beneficial. With wireless contollers and all. Obviously it would have to be as close as possible to the original systems. I'd buy that too.
I'd like to see this too. If I remember correctly, the 7800 already can play 2600 games, so that's 2-in-1 right off the bat. Then you'd just need 3 cartridge slots. Slap on the "wood finish" of the 2600 and I'm sold.