#2 Perfect time and scenario
Brock Lesnar, with the aid of Baron Corbin, decimated Braun Strowman in a squash match to win the vacant Universal Championship at Crown Jewel. Given the backlash WWE is receiving over this creative decision and Lesnar being a part-timer himself, WWE has a far better option in the form of John Cena.
Crowning Cena as the 17th time World Champion would not only fulfill that long lasting wish of his fans and be a welcome change on Monday Night Raw, but it may also help WWE by boosting the down and decreasing viewership.
Given Cena has transitioned himself into a part-timer now, a long stint as the WWE Champion seems meaningless and looks highly unlikely. And with Braun Strowman involved in a temporary feud with Baron Corbin, this 1-2 month window in 2018 before the Royal Rumble pay-per-view might be the best time for Cena to dethrone Lesnar.