Steve Williams and Sean Morley for me.
Morley can easily be marketed as the new generation, he'd be young, a heartthrob and should still be around a heavyweight as well in size so could use that as an advantage. Face he'd be great as a young upstart, heel he could already come in cocky or eventually turn on Flair as he wants to be the man.
Williams is tough as old boots. If it goes down the face route you can easily play it as Flair wanting to give someone a chance at the big time who's long deserved it and just wants to fight people, he's got the heart of a horseman. If they go heel well he just wants to fight people and while he may never be "The Enforcer" having a technically superior worker who likes to fight he'd be a great mid card foil for some of the up and coming faces, he'd be great against Dustin for example.