Dataminers who leaked Splatoon 2 updates have also said there are references to two or three more emulators inside the updated NES app, so two or three more systems might be added this year. My best guess is we'll get SNES when the next Direct happens and the GameBoy line (original/Color and Advance) later this year. If Nintendo bring SNES out and confirm more systems for this year I'll actually get a subscription.
N64 is a possibility, but I think the nature of the app rules out GameCube. The classic apps download the whole library of games - that wouldn't be feasible with GC, given the size of games and Switch's limited storage.