Taker/Wyatt can still happen, and it could be what puts Bray over huge with a win, especially if it's Taker's last match. That would make Wyatt into an icon right there, and solidify him at the top of the card, as the top heel if nothing else.
Not really, the big Undertaker match was the one that broke the Streak. He'll get maybe 20% of the rub from ending his career that they wasted when they gave Brock the Streak just for it to give him no extra credibility.
But yes, still Undertaker, but it's too late. Undertaker will crumble into a pile of dust if he wrestles again.
I would say Wyatt vs Ambrose but Ambrose is never going to be used as anything but a low card geek and Wyatt deserves better than to be stuck with a low card geek. Elevate them both to the main event and we'll re-visit the idea.
Bray Wyatt vs Daniel Bryan, for the matches and the experience he'll get in the ring.
Most of all, whoever is WWE CHAMPION. I don't buy for even a second that Bray Wyatt is a future WWE Champion, but it NEEDS to happen. It NEEDS to.