Will N64 games ever be playable on Dolphin or will it just stick to Gamecube and Wii games only?
Dolphin is not going to support N64 emulation. But it does support running the Wii Virtual Console releases of N64 games.
You can run Wii homebrew emulators such as Wii64 and Not64 on Dolphin. But keep in mind only a few games run decently without major graphical bugs.