I would prefer it over a 3D donkey kong game. However we are all going crazy on figuring out what Retro is doing right now. At this point we all have crazy expectations on what it is.
Anyway I doubt they have been working on Smash Bros. Tropical Freeze finished in 2013. A year before Smash 4 came out. So unless they were working on a Smash Bros long before the 4th one finished I will make it up as highly unlikely.