I think the best possible matches that make sense from a storyline standpoint are these (Not including the possibility of Rock or HBK wrestling):
Finn Balor vs. Kevin Owens (c): Balor returns at Rumble, wins, challenges Owens for the Universal title. Brings Balor back into world title picture against the man who won it right after him
A.J. Styles (c) vs. The Undertaker: It's only right, and I think they can still save Cena/Taker for next year since it's looking like Taker's sticking around. Make it for the title too
Triple H vs. Seth Rollins: Bring closure to the feud, complete Rollins' face turn
Brock Lesnar vs. Goldberg: Now I would rather see something else, but this is what's most likely gonna go down
Charlotte vs. Sasha Banks vs. Bayley: They did 3/4 of the Horsewomen last year, makes sense to do it again this year but with Bayley replacing Becky
Roman Reigns vs. John Cena: Match that should have happened while the brands were still together. I don't know what the story would be about but make this the big interbranded match
Shane McMahon vs. The Miz: Addresses Miz's beef with SD management/Daniel Bryan. Maybe on SD Miz puts hands on DB and Shane wrestles Miz in DB's honor
Dean Ambrose vs. Samoa Joe: Bring Joe up to Smackdown, most sensible feud to inject him in right away
Interbanded Ladder Match for the US Title: Jericho (c) vs. Cesaro vs. Zayn vs. Braun vs. Ziggler vs. Orton vs. Corbin vs. Kalisto
You also got New Day, Enzo & Cass, Braun, Bray, American Alpha, Crews, Rusev, and the Cruiserweights on deck to fill out the rest of the card