I mean Super Mario 2 is the correct answer. The other 3D games aren't happening bet on that, and Yoshi's Island is not even a Mario game. Mario Land could happen but why would that be in the "Extra Game Styles" section? SMB2 is unique enough not to conform with the other styles. If its not Mario 2, Ill be really sad.
Mario Bros 2 USA is the would be the best choice for uniqueness,
I guess Yoshi's Island could be cool but that's considered a Yoshi title now instead of a Mario one. Mario Land 2 or even 1 would be neat but more can be done with Bros 2. Mario 64/Galaxy/Odyssey, eh could be cool as well.
Any choice would be great, I'm also guessing these are going to be DLC...
In the original game, while I loved the fact that you can change 'on the fly' the game style of one of your levels (seriously its really cool to see) I'm happy to sacrifice this ability for more styles to be added! So I'm happy about this 'extra' thing, giving us this means we'll hopefully eventually be able to make and play custom Mario levels from ANY Mario series!
I can see this being DLC for sure.
Personally, I guess Mario 2 (USA) should be next as its very unique and would complete the NES ones nicely.