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.
While 1GB of storage is enough to hold at least 64 N64 games (based on the average size of 16MB per game), which is probably more than we will ever see, with the GC you'd be lucky if you could fit one game in the same space. Of course they could introduce some heavy compression, streaming etc, but I'm pretty sure this isn't something that we should be worried about as most likely we're not gonna see a single GC game released for the (current) Switch, apart from the fully paid remastered versions maybe.