Totally! I love these ideas!
Especially the multiple skins, I've seen that in Kung Fu Chaos and it's a good feature that prevents bickers over what player gets to be what character. Although, I think with this feature ought to come with more dialogue, as four players with the same character and voice clips will get very annoying quickly.
As for boards, I'd LOVE for there to be new ones, as this means Maginaryworld is full of worlds. One board I'd love to see is one that involves music (much like Music Plant from Sonic Advance 2).
If I were to also add more features, I'd definitely include additional unlockable characters that are not part of story mode; ranging from popular characters like Shadow and unpopular characters like Manic from Sonic Underground.
This topic coincidentally relates to a post I made on Reddit and Amino. I was asking about whether or not people would like to see Shuffle get remastered or have a sequel to make it better, and I think until a continuation of the plotline can be done, a potential remaster is in order. I also think that along with updated graphics, the game probably needs to be worked from the ground up; receiving an overhaul of updated dialogue, animation, and, of course, reworked gameplay mechanics (In my opinion, the music and sounds are perfect as is). Maybe it's alot for a simple port but I really love this game too much to let it not get attention.
I think you've pretty much added the right amount of stuff but I think a game like this should never stop growing, hence why I had so many ideas, myself. xD