I think it would be very interesting to get some emulators on the 3DS, Since the screen is exactly 240p. NES, SNES, Genesis games will look very, very good on it; maybe we could get a Retroarch port on it and get some stuff running. An actual 3D VirtualBoy emulator would be nice too since Nintendo refuses to release VB Wario Land on the eShop. I'd also like to see region locking bypassed.
I'm not really interested in piracy right now. Back on the DS days I used to pirate everything and I'd find myself with piles of games on my SD card and no motivation to play them all. Now that I'm buying my games, I feel very inclined to 100% one game after another to get my money's worth. It's satisfying. That being said, I'd like an exploit to play backup games so I could look back and play some forgotten gems a few years down the line.
EDIT: Also, not to mention an exploit like this opens the door for some other nice things, like better documentation and understanding of the 3DS hardware, which would lead to emulators. It would be nice to see 3DS games in high resolution on the PC.