They stick mostly random faces on their Wrestlemania production trucks. They don't necessarily mean anything. Kofi Kingston and Wade Barrett were two of the faces they used for one of the production trucks for Wrestlemania 29, and in what spot on the show did they end up being booked on? One was on the pre-show, and the other wasn't even on that.
As for whether I'd want Undertaker to wrestle another match or not, part of me wants to say No since he's become all about the Streak at this point and the Streak is dead, but the other part of me wants to say Yes. Taker is one of my favorites and him having one last triumphant moment and victory at Wrestlemania would be a great sendoff, especially against a profound enough opponent. I'm thinking either Bray Wyatt or a double-retirement match with Sting sounds good.