It's something I've been hoping for a while now, and Nintendo have officially confirmed that Gamecube games are heading to NSO once the Switch launches.
Nintendo have announced that The Legend of Zelda: The Wind Waker, F-Zero GX, and Soul Calibur II will be launching first with other games such
Pokemon Colosseum and XD listed as upcoming titles is very surprising. Does this open the door for the rest of the main series to come to the other NSO apps?
Those two aren’t actually considered main series Pokémon games. They’re the only side games that can catch and train Pokémon that can be traded into the main series games. Pokémon Stadium is a similar release that’s already on the Nintendo Switch Online N64 app.
It remains to be seen whether Pokémon Home gets an update to support these GC games.
I very much doubt the main series games will ever be added to the NSO GB/GBA apps. It seems likely enough that they’ll rerelease the classic games in some form on Switch next year for Pokémon’s 30th anniversary (similar to how 3DS got the GB ones for the 20th in 2016), but I fully expect that the release will be under The Pokémon Company’s terms rather than a part of NSO. Either as part of the Pokémon Home subscription or sold on eShop.