Mike wasn't as well-known, so I can't imagine him coming back, even if it is for a special occasion...besides, from what I've seen of him, I don't think there was anything unique about him character wise...and his voice really annoys him...and as far as the running gag of him always being hungry and having snacks with him goes...not one bit amusing- that's what a certain googly-eyed, shaggy blue furred monster has always been around for!
(So, as you can guess, I won't be surprised if he doesn't show up)
Savion was definitely more well-known, and had a better character, so if he decides to appear, cool.
But getting back on subject of humans that will be (or are planning to) participate in this special, I'm sure it will be an outstanding turnout. If I had to choose who I'd like to see on there...if there was a way to bring people back from the dead, Mr. Hooper, David, Olivia, and Mr. Handford would be my picks...but at least they'll be there in spirit. All in all, any humans that will be able to show up will put a smile on my face!