You're never getting a Mania in the next 5 years that's all new talent. They can't even produce good raw ratings, so there's no way that they can carry an entire WM.
WWE didn't build any superstars for what you're taking about.
Bringing on Rock/Austin/HBK/whoever does fuck all because it's been done so many times now, people know they're gonna have some five minute spot and then that's it. It barely moves the needle on a rating any more.
For as much as everyone here still jerks it over stuff that happened in 98-99, they forget that all that groundwork was laid in 97 when the ratings were down. Better to experiment and build when there's less eyes on it. Give Roman his run now, and if he can hang, stick with him. If not, you get the belt off him at RR and you're not shitting up a WM title match with a guy that can't hang. Let guys change their shticks and see what happens.
But relying on some old man to come out and work a match so people can go "man, he looks like he's on roids/can't go any more/was better 10+ years ago" is a waste. Doesn't matter who is the old man in question. If you haven't been actively involved in the show for over a decade, fuck off. Not even for a legend segment or whatever, unless you got into the HoF.
The only way I would use the Rock at this point is if you used him for a Roman heel turn. And not in a match. There's ways for Rock to be involved without taking up a spot on the card. Hell, Austin did it weekly in the early aughts.