I would love to see Suffs do "Great American Bitch", it's the song that's the most fun in the show.
"Keep Marching" is the song I expect to see. I get wanting to use a song that utilizes the entire cast, but I actually preferred the version Shaina Taub did solo playing the piano on Kelly Clarkson to the version in the show. More is not always better.
For the most part, I'll only comment on what I've seen so far.
BEST MUSICAL
Hell's Kitchen: 'Kaleidoscope' seems like the fan-favorite choice, but I wouldn't be surprised if they go with 'Empire State of Mind'; it would include all four nominees and it's certainly popular/relevant
The Outsiders: Haven't seen it yet, but 'Great Expectations' seems to be their "signature song"
BEST REVIVAL OF A MUSICAL
Cabaret At The Kit Kat Club: I could see them doing a brief segment of 'Wilkommen' before segueing into 'Cabaret'; but if they only do one number, I hope it's the latter
Merrily We Roll Along: 'Old Friends' would be ideal, but I could maybe see them going with 'Now You Know' or even 'Our Time'
Tommy: Probably some kind of medley, but if just one song then either 'Pinball Wizard' or 'I'm Free'
(I honestly can't see Gutenberg! performing at the ceremony, but I'll be pleasantly surprised if I'm wrong - if they do, the only option would be 'We Eat Dreams' and having yet another surprise guest as the Producer)
NOT NOMINATED, BUT COULD/MIGHT PERFORM
Back to the Future: The Musical: 'It Works' would show off Roger Bart; but if they don't want to do that, and don't want to showcase any of the new songs, they could just go with 'Johnny B. Good' for familiarity
The Notebook: Maybe Joy Woods' big number in the second act, but I would think they want to showcase all three Noahs and Allies