Well there are quite a few options but the truth is we might not know yet, Cena fought Wyatt at his first Mania and then fought Rusev at his first real Mania match, so it might be a debuting guy.
Cena/Taker would have been a huge draw, on the level of only a few other matches but that time is gone now. The draw would have been the fans not knowing who was going to win, if Cena would end the undefeated streak of the Deadman...but it's over. So I think in a lot of ways Taker matches at Mania have dropped in importance and drawing power. My favorite guy so I'm not badmouthing him, just how I view it.
Cena/Bryan was my pick for what Cena would do this year, a face/face feud involving the Bellas which would have explained why they kept using Bryan pops on Brie, plus with Brie turning heel it could be interesting to see how the WWE would work it. Plus it would give Bryan one of the highest matches at last years Mania, sadly though I don't know if it would work as well this year. Bryan has been pushed slightly down the card and with him not being out till the Rumble there are far better options for both guys.
Something we aren't really considering is Cena/Orton. I know we as a community wouldn't be behind it but the WWE consider this one of the best feuds of all time and if you look back, they've never fought one on one at Mania...why haven't they? So expect it to come soon especially if they are serious about selling the stadium out with lots of big name matches.
Cena/Reigns is very much an idea but I think the best time for this would have also been this year, but with the Reigns backlash this match has bad all over it.
Cena/Rollins could be an interesting sleeper pick, imagine Rollins has a great year, has the title almost the entire year. If the WWE could work it, I think this would be the perfect match for a double switch, I don't know why but I think that. Would be even better if Seth could get the clean win.
I'm not quite seeing Cena/Austin as an option, of course they could always throw Rock back at Cena
If I had to make a serious guess, just one guy who I would put money on it'd be Bryan I guess. Not too confident but he's the closest thing to sure I am