Current...

7800 - just need F18 Hornet to complete all games
Neo Geo Pocket Color - just need Neo 21 and Dive Alert Becky to complete all UK released games

Longer term

C64 carts... as many as possible from the small number I still need
C64 Chalkboard Powerpad and the 5 carts for it
Any Vic20 carts I still need

Very long term

Maybe start collecting for 5200