That's coming with Mario Kart UltimateHonestly there's no reason to work on a new Smash game anytime soon, just use Ultimate as a foundation and keep adding DLC for years to come. That's what Nintendo should have done with Mario Kart 8.
Also, I'd hope he could juggle Smash DLC and a new project if he wanted, but like everyone else is saying as long as the man keeps a good work/life balance he should do whatever he wants.
If he's doing what he loves that's all that matters. You can tell he loves smash and he wants to keep ultimate going as long as he can.