When I was just starting to program for the PSP, I envisioned first doing a port of fMSX, then a port of an emulator specifically for emulating arcade versions of Mortal Kombat games 1-3. Since I don’t think any emulators other than MAME support the games (and MAME’s source is a tangled mess), I doubt that phase will ever come to fruition.
Most recently I’ve also considered porting older games, however, and if someone else doesn’t do them first (I hope they do), I just might do a port myself. Two obvious choices right now are OpenTyrian, a rewrite of Tyrian, and Wargus, the Warcraft II-compatible RTS engine based on Stratagus. I prefer not to port these projects myself, as they’re both based on SDL, and I happen to have an inherent dislike for hardware abstraction frameworks.
If you’re looking for an interesting port project, both of these games should be a welcome distraction from the many flavors of DooM already ported to the PSP.