Smash Bros Melee- new characters, maps, and new platform-based single player modes based off Mario, Zelda, and Metroid.
Tenchu 3 or Kurenai- Level editor. I had great fun with part 2's editor despite its limitations. I would love an editor for a more complex Tenchu. Tenchu multiplayer is dull in comparison.
Final Fantasy Tactics- Yes, its almost seven years old. I don't care. A true sequel would be better, but I would even love just having more classes and missions in the original.
Donkey Konga-much anger if no expansions release here. I love the game, but the current song selection leaves much to be desired. More latin and classical music please. More Nintendo music too.
Super Mario Advance 4- One of the greatest platformers of all time, yet theymucked up all the bonus content by requiring you to buy it seperately and then not releasing most of it in the US. I would buy a cartridge with all the new levels available in Japan and the ability to unlock the colored switches and cape/boomerang in SMA4. Either that, or a Super Mario Bros Remix with the best levels of all the games combined with many all new levels.