Daniel Bryan - He will either be thrown into the main event after a non-finish at Fast Lane against Reigns or (and this is the one I would rather see) they'll put him against Dolph. I would say there's also a small chance he could face Cena because Rusev could always beat Cena and move onto Ryback at Mania. They teased that match before Ryback got fired. Not likely, but as I said, there's a small chance.
Dolph Ziggler - My 1st choice would be to see him go 1 on 1 against Bryan. I've read the rumor that he and Ryback will become a tag team and be part of a triple threat or fatal 4-way for the titles. There's a chance he's just thrown into the Andre Memorial battle royal if they have it again this year.
Dean Ambrose - I think it's most likely he has a rematch with Bad News Barrett whether he wins or loses at Fast Lane. The only other opponent I could see for him is Sheamus if he turns heel upon his return.
Sheamus - All kinds of possibilities here. I think Ryback is the likeliest opponent because I do see Sheamus being a heel just to shake things up. Ambrose would be my next choice. If he remains a face, there's always a chance he finally gets his rematch with Rusev. Bad News Barrett is another option. I think he or Ryback will win the Andre battle royal if they're in it. There were rumors he'd go up against Daniel Bryan, but those apparently have been scrapped.
Ryback - As I said, my best guess is we see Ryback vs. Sheamus. Maybe Big Show just so we can see him get Shellshocked (though I don't know if they want to risk him botching it - remember Tensai?). Tagging up with Dolph or entering the battle royal are potential ways they could go. I also wouldn't rule out him getting a title shot against Barrett or Rusev.
Luke Harper - Definitely in the Andre battle royal if they have one. Maybe part of an 8 or 10-man tag if they have The Authority's guys take on Dolph, Ryback, Rowan, and others.
Bad News Barrett - My best bet is a rematch with Ambrose. A small chance he defends against Ryback or Sheamus. An even smaller chance he could defend against Jericho. I don't think he's taking part in Mania, but if he did, this is where I'd use him. Jericho gets his 10th IC Title and loses it back the next night.
With all that said, this is how I see the Mania card shaping up. I'm really hoping they don't make the main event a triple threat again. There's nothing wrong with having one every now and then, but we had one at Mania last year and we just saw one at the Rumble. I'm not a fan of Reigns, but in my opinion Vince should stick to his guns this year and not cave for the Bryan fans again. He gave them what they wanted last year. If they boo, so be it. True wrestling fans would be happier with a Bryan/Ziggler classic.
1. WWE World Heavyweight Championship: Brock Lesnar (c) vs. Roman Reigns
(followed by a Seth Rollins cash-in)
2. Daniel Bryan vs. Dolph Ziggler
3. Undertaker vs. Bray Wyatt
4. Sting vs. Triple H
5. Divas Championship Fatal 4-Way: Paige (c) vs. AJ Lee vs. Natalya vs. Nikki Bella
6. US Championship: Rusev (c) vs. John Cena
7. Randy Orton vs. Seth Rollins
8. Intercontinental Championship: Bad News Barrett (c) vs. Dean Ambrose
9. Andre Memorial Battle Royal: Winner -Ryback
10. Career Match: Goldust vs. Stardust
Pre-Show
11. Damien Mizdow vs. The Miz
12. Tag Team Championship: The Usos (c) vs. The Ascension vs. Cesaro & Kidd