I want them to take this alt costume system they have working and sell me DLC.
I think Dry Bones and Paratroopa could make their triumphant return as Koopa alts. It would work perfectly fine, animation wise, yea you'd probably lose Dry Bones falling apart or spinning his head, and Paratroopa wouldn't properly fly, but I think they'd make nice alts and Dry Bones is a super popular MK character.
A pack of red koopa, paratroopa, green paratroopa, dry bones and red dry bones would be nice.
I think they're done with stage DLC, but I could see them continuing to add new vehicles and characters.
Bringing the original guest character, ROB, back would be awesome (with both US and Famicom colors). Plus Diddy and Bowser Jr. And adding cool characters like Kirby or Hammer Bro. And alt toad colors. And Fire/Ice Mario and Luigi. And the Dry Bomber and Wario Bike. And more and more, DLC for MK is great.
I really hope this isn't the end. There's so much more they could do without making new courses.