Adele, Beyonce, Bieber, Drake, Rihanna (if she bothers to actually show up this time), Sturgill Simpson, Lukas Graham?, Chance the Rapper?, Twenty One Pilots?, Maren Morris?, Miranda Lambert?, etc.
I could see Ed and Bruno performing.
edit: Praying for Bey and Adele to do a duet though.
Basically Guaranteed:
Adele
Beyoncé
The Chainsmokers
Chance the Rapper
Drake
Ed Sheeran
Justin Bieber
Kelsea Ballerini
Maren Morris
Sturgill Simpson (although he said he might skip depending on when his wife gives birth)
Tom Petty (or tribute to Tom Petty; he's Musicares Person of the Year)
twenty one pilots
Other Likely Possibilities:
Ariana Grande
Bruno Mars
Demi Lovato (with Brad Paisley?)
Kanye West
Lukas Graham
Miranda Lambert
Pentatonix and Dolly Parton
Rihanna
Less Likely But Still Possible:
Barbra Streisand
Bob Dylan
Brandy Clark
Kelly Clarkson
Kenny Chesney and Pink
Lady Gaga
Lorde
Lori McKenna (with Tim McGraw?)
Shania Twain
Willie Nelson
That's not getting into the collaborative performances (I think Drake, Bieber, Beyoncé, Rihanna, and Sturgill with have guests).
Can't decide if the George Michael tribute should be performed by one of his friends (Madonna, Annie Lennox, etc) or by a gay artist (Elton John, Sam Smith, Melissa Etheridge). But one of them will do it.
They'll probably do a Prince tribute, since he died after last year's ceremony.
Also totally here for someone singing "Good Morning" to tribute Debbie Reynolds. I don't really know who they would tap to do that, though.
Rihanna should perform KIB with the set that she was supposed to do last year maybe medley with LOTB
The Weeknd should perform Reminder/IFIC
Bruno's next single
Katy's lead
Ariana also better perform Sometimes/Everyday or just Everyday
and I want Gaga to perform too maybe a tribute or A-YO or MR or whatever
Beyonce def will perform too