#1 Kofi (or the New Day) puts something on the line
Maybe the best and most effective way for Kofi Kingston to get his match for WrestleMania is the simplest. One more passionate promo from him, Big E and Xavier Woods could do the trick. They agree to put something on the line at WrestleMania. It could be either Kofi himself, or the New Day as a whole.
After all, Vince McMahon would love to have a WrestleMania world title match which has severe consequences.
Kofi Kingston could agree to put his career on the line but since another couple of stars are having their final match at WrestleMania, a better option would be for the New Day's future to be in the balance. If Kofi loses, The New Day must disband. This would drive the stakes up to a ridiculous amount, with the WWE universe pulling for the challenger even more. The New Day must never break up and they wouldn't, even after this WWE Championship match.
This is because only one man can stand tall at WrestleMania with the WWE title in his hand. And that man is Kofi Kingston.