RetroArch would be nice since it's many emulators in one so you don't have to exist and restart the hack each time you want to change emulators or games.
PS Vita 1002 - OFW 3.51 Rejuvenate
PS Vita 2002 - OFW 3.60 HENkaku
Vita TV - OFW 3.60 HENkaku
I voted for many things, but the most important one wasn't there: SDL.
It isn't usable by itself, but it's used in many open-source projects, and porting SDL to Vita would, in turn, make porting said projects to Vita a lot easier.
I especially want to see Vita ports of (SDL-based open source alternatives of) these legends of 2000's:
Fallout 1 & Fallout 2 - Falltergeist
Heroes of Might and Magic 2 - fheroes2
Heroes of Might and Magic 3 - VCMI Project
Baldur's Gate and Icewind Dale series, Planescape: Torment - GemRB
Turn-based games are perfect for Vita, its 960x544 resolution is pretty close to 640x480/800x600 resolution of original games and they weren't that demanding in regards to hardware (being 2D games).
Well, Infinity Engine (GemRB) games are "real-time with pause", so they won't be as easy to control as truly turn-based games (all but the last line in the list above), but they are still awesome and pretty moddable.
It would be so great to see more AAA games on Vita, especially after Sony have officially refused to give them to us...
Retroarch and SDL should definitely be ported to the Vita.
I use retroarch in the nvidia shield and it is a great frontend with many options and cores (emulators). The difficult part is porting or writing the retroarch drivers: video, audio, input and gui. But after that its quite easy to build and add the existing cores. Maybe the PSP port should work as basis.
Personally I would appreciate emulators with any kind of netplay, ad-hoc or even X-Link based, and the whole homebrew world lacks it greatly. That was a huge off PSP limit, but belowed Vita with native recources should get job done in a thick.
Please, for any dev out there messing with nes/snes/gba/md/psx emulation - take this side of development into consideration.
Best regards.