Would it have been a far-fetched idea to think that the Vince/Shane angle could've involved the main event. Vince backs HHH and Shane backs Reigns in the main event with the winner getting what they wanted...Reigns winning means Shane gets Raw and vice versa.
Shane being so over could possibly rub off on Reigns?
Reigns winning would have more weight than just winning the title back?
Then you can use Undertaker something else. Does Shane even have any in-ring game left?
Shane vs. Undertaker just seems thrown together...why would Undertaker work for Vince?
Adding it to the main event would kill the story dead and there is no rub that will get Roman over.
They also wanted multiple giant matches at this mania so making a seperate match for this made perfect sense.
It also creates the biggest stakes dot a non streak match they can outside of a retirement angle(and he isn't ready to retire so they can't really use it yet).
As for the match. The gimmick protects Shane's in ring ability. He just needs to take a beating really and hit taker with some weapon shots.
It's going to be booked as a Taker squash with Shane getting a hope spot with weapons (why HIAC works here) until they go into the overbooked finish that I guess will involve Vince and I think Cena (but it could be a number of guys).