I think mostly everyone would prefer they go the logical route of Styles/Cena. It makes so much sense in story and would be the best main event we have had in years. However, I can't imagine them going away from Cena/Undertaker - it is the last opportunity they will probably have to do it.
So if Styles somehow retains against The Undertaker at the Royal Rumble, I don't think there is any better or logical contender than Ziggler. It would line up with his story arc on SmackDown - how he's tried so hard to prove himself. Granted he's won world titles before but never the WWE Championship and it would be at WrestleMania. Plus it would definitely be a great match. I'm not the biggest Ziggler fan, but right now this is what I'm hoping for after Styles/Cena.