#3 Play up their history
Believe it or not, The Undertaker and Bray Wyatt have a history together and there might just be another chapter to be told there. Interestingly enough, WWE has been using Wyatt's history with other Superstars as justification for why he attacks them, which could end up paving the way for The Phenom and Wyatt to finally meet again.
In fact, why not have The Fiend outright try to goad The Undertaker into a match? Not only would it be a unique way to eventually bring the two Superstars together, but also allow The Fiend to play those mind games he does so well. Heck, The Fiend could even outright state that he wants to hurt Undertaker for what he did to him at WrestleMania 31.
In the end, the point is that these two need to meet inside the squared circle sometime soon. Fortunately for WWE, doing that could prove to be relatively easy due to the history the two Superstars have with each other and they should really explore that. Either that or disappoint fans with something less awesome than the dream match that would send everyone home happy, irrespective of the outcome.