They've certainly had enough time to make a new game. I think we're looking at a Splatoon 2 style sequel - re-used the engine and assets (in some cases) but created new content.
At first this is what I thought it would be. But considering the short amount of time it took to make wiiu/3ds, I'm starting to think that everything might be new. It took them around 2.5 years to make 2 new smash games from the ground up.