Sure enough, it's safe to assume that Prince had numerous projects on the go these last few years. My guess, and not suggesting that any of these projects were finished:

A 2nd 3rdeyegirl album: Plectrumelectrum was recorded months (over a year?) before it was released. Surely they have a lot more music recorded. There's evidence of more experimental music like Moondrops, and 3rdeyegirl music with additional orchestral instrumentation from the promo video with Donna.

Hitnrun Phase 3 / Free Urself: A collection of songs with the feel and production of Free Urself and If Eye Could Get Your Attention.

A possible piano album, an album with MonoNeon, an album of songs with orchestra and guitar exchange as described by Nelson the StringGenius man? (These are less likely I guess).

And then of course, there's anything and everything else that Prince might have recorded!