I predicted something from the Mario universe right before Bowser, because it had been the longest since we'd seen anything from that series. The new series that hasn't gotten a Smash update in the longest time is Kirby. However, if the only two Kirby characters are Kirby and Metaknight (and let's face it, other than King Dedede, that's really about all the options there are), I don't think Sakurai would want to do Metaknight and "finish off" the Kirby updates here and now.
So, my vote is for one of these Kirby-themed things: 1) at least one of the animal helpers from Dreamland 2 and 3 as assist trophies, 2) a level with Wispy Woods since that's a Smash staple by now, or 3) Kirby music that will own everyone in this thread--and many people out of this thread.