#1 To make Big E a main event star
There is no doubt that if you look at the New Day through Vince McMahon's eyes, Kofi is not the person he would like to push to the top of the company. Vince famously looks for how big or tall someone is to push them rather than talent so out of the New Day, Big E is the person who will most likely get pushed to the top in the long term. Big E has been a lovable giant during his time in the New Day but he could be so much more if they let him.
Kofi is, of course, a lovable babyface and Big E could be the giant heel who attempts to dethrone Kofi. The story is simple and one that has been done a million times before by WWE. Have Big E turn heel on Kofi and beat him for the WWE Championship.
While this may anger some fans of Kofi it is the logical next step in both men's careers. Big E is four years younger than Kofi and has a lot more marketability at least for what the WWE looks for. However I don't think he should turn on Kofi immediately and instead wait until they can have a match on a big show.
Kofi winning the title at Mania and celebrating with his New Day partners Xavier and Big E will be a great moment. So I think Big E should turn heel around SummerSlam time and have a big blowoff match at the PPV. Not all stories have to have a happy ending and having heel Big E come out on top over his former partner is not only a great story but makes a new main event heel for the company this summer.