How I would book Cenas title loss and subsequent face turn

Very simple.

Between now and SNME in December keep him heel but tease the doubt we’ve already seen.

At the same time, build Seth Rollins up to be an absolute tyrant

Cena drops the title to a deserving opponent and then Seth goes to cash in. Bell rings and Seth goes for the stomp but Cena takes Seth and his goons out. New guy retains, Seth’s cash in fails.

Cena gives the new guy a sale and then leaves the arena to no fanfare whilst the new champ celebrates.