Board ideas
Just some things I thought could be boards
Rainbow road- A rainbow road like mario kart would be a pretty cool inclusion. Doesn't have to be based of a existing one though. The main issue might be it's to linear especially if it's similar to say N64. I'd say definitely add split paths maybe add a moon like in 7 then split that into 3 paths maybe there could be a chain Chomp space there that causes you to lose coins. One path could also be longer but have a boo or shop.
Flower Kingdom- They could also have one based of the flower kingdom in mario wonder. Something that wouldn't change gameplay but make it more like wonder is instead of stars it could be wonder seeds and poplins selling them. The petal isles would connect the other areas together but just so it's more like wonder you spawn in pipe-rock plateau instead. A few poplin shop locations could be shop locations. Light switch mansion could be where boo is maybe somewhere else also. You could also race against wiggler for coins but you can lose some also.
Paper Mario- They could add a board based off paper mario with unique designs for everyone and items and stars. I'm not sure what they could base it off of maybe something in the first game. I don't know much about paper mario I just thought of this.
A board for each koopaling- This is really biased considering I actually love them. But in story mode or basically whatever single player mode there is you can unlock their board or maybe even them. For what to do to make them each unique I'm not sure though.