I do see Shantae very likely as an AT. as other people have mentioned, Sans would be cool too, and quote, to complete the indie pantheon.
saw a video teh other day about "reasons" why shantae was a playable character in smash.. and honestly 90% of the video was massive straw grasping, but the 10% related to some wayforward stuff seems convincing. the problem is that theory works both, for her as a playable character, or for her as an assist.
I also was hoping to get Guile as a Street fighter assist :0 and a couple more of returning one like phospora, saki and the like.