"Skins" is kind of a very basic way to describe it. Like if I'm going with SMB2 stuff I want to be able to stand on a Shyguy and then throw him. That's a different gameplay mechanic than how anything works in SMB1 or pretty much any Mario game except SMB2. Art assets aren't really what I'm looking for. I want to be able to take any element from pre-existing Mario games and put it in the level and it acts like it would in its source game. Koopas for example work differently in SMW because they shoot out of their shells. If it's just putting a Koopa in that always acts like SMB1 Koopa but you can make it look like a SMW Koopa, there's not much to that. It's cooler than being limited to just SMB1 or NSMB style visuals but it's superficial.
I believe 3d world actually handled this by introducing some sort of goomba subspecies called a 'galoomba' that looks and behaves like the ones in Super Mario World so they could do something similar. Either way considering how much they put into the e-reader levels for Mario 3 (seriously, they had levels with charging chucks and those penguins from yoshi's island for god sake) I think it's safe to assume we can expect to see some things taken from other games,
But the fact that the skinny mushroom exists(I hope they design a counterpart that reverses that effect, it could be interesting to create levels based on constantly switching between Mario and Luigi's physics) should prove that they aren't beyond integrating new things.
Although lets be honest, there were no Hammer Brothers in Super Mario World, what we got was the AMAZING FLYING HAMMER BROTHERS instead.
I am curious if they decide to include enemies from outside of mario (which they may, I'm honestly under the impression that they basically had just gotten the general editor engine going and had to scrape some stuff together) how they will handle the changing art style. SMB is honestly a pretty poor option to have for this kind of thing I mean it will be hard to include some of the more complex enemies in later mario games like a Charging Chuck and have it mantain that sort of fast paced feel that chuck has with only two frames of animation.