I don't know about that. I think Nintendo always intended to make Captain Toad, but the levels in SM3DW acted as a testbed for the mechanics. Turning it into its own game just seemed so obvious.Nintendo does this a lot.
Mario land 3 on Gameboy was so different (hint: Mario isn't in the game) that Wario got his own franchise
Iirc, the captain Toad levels in super mario 3d world (wiiu) were so popular it spawned captain toad spinoff
Edit: Unreal. It originally had some deathmatch levels thrown in as bonus to try to play with friends on that internet thing for those who had it. It became unreal tournament,
(I presume quake 3 had er origins but I honestly don't know for sure)


