As far as homebrew goes: 3D games and movies/movie players are a given. We'd be looking at advanced SNES/GBA/MAME and maybe N64/PS1 generation emulators. Also, if full control is established, perhaps custom camera homebrew (Custom AR cards/games?), homebrew that utilizes the gyro, the pedometer, the improved wifi functionality the 3DS has over the DS, perhaps even the Infrared could be used for something? (Universal TV remote control? That would be crazy...is that even possible?) No more playcoin shortages, no more shitty browser.
Lots of doors would open, and things would definitely get interesting. There might even be another homebrew contest held right here on the Temp someday.