So let us assume this is going to happen. We will have one last Undertaker match... again.
Let us also assume it will be John Cena vs Undertaker. This will be his official final match.
With that being said...
How would you like this to be booked? How do you expect it to be booked?
I think if this is going to happen, it is best he comes back as the biker version of himself. Since he did technically retire at the "dead man" it would make no sense to come back like that. This is a good way to make a pretty cool entrance as well. As far as booking goes, him losing two years in a row would not seem right so maybe keeping this a standard match, not a retirement match would be best. Let him go out on a win over Cena.