Would it work to have the guy who plays George Sr on Young Sheldon appear as George Jr on TBBT? (And, yes, I know that he’s already appeared on the show as a completely unrelated character.)
The even cleverer thing would be to have the father of Montana Jordan (who plays George Jr.) play the grown-up version of George Jr. That would be appropriate because the daughter of Laurie Metcalf (who plays the older version of Mary Cooper on The Big Bang Theory), whose name is Zoe Perry, is the one who plays the younger version of Mary Cooper on Young Sheldon. Unfortunately, the father of Montana Jordan, whose name is Tony Jordan, isn’t even an actor.
Leonard’s writing a book lays the pipe for future stories.
Was that a Timmies cup on Stuart’s counter at the comic book store? Do they even have Tim Horton’s in California?
I didn’t know he was writing that kind of book! 
Sure, that works.
Actors have played two different characters on other shows - Harry Morgan on “MASH”, f’rinstance.
Setting up new characters on YS also lets them appear as adults on TBBT, like the kid’s Vietnamese friend (Pham?). Sheldon’s twin sister Missy was on an episode long ago, and she certainly could return.
It wont’ be I telling you that you’re wrong