The Republicans have long acted in accord with the "next in line" principle for their Presidential nominations, but just who the next-in-line is sometimes varies: occasionally it's the runner-up in the previous primary (Reagan), occasionally it's their Congressional leader (Dole), but Republican Vice-Presidents and Vice-Presidential nominees, actually, have not had much success in becoming Presidential nominees. With the exception of George H.W. Bush, the last former Vice-Presidential nominee on the Republican ticket to attain the Presidential nomination was Bob Dole in 1996, and there was a twenty year gap between his Vice-Presidential and Presidential nominations.
I think, for now, Ryan has to be assumed to be the frontrunner for want of any other candidate. I do not at all think he will be the nominee. I think it's especially unlikely he'll be nominated if the Democrats put up Biden because of how badly Biden beat him in this year's Vice-Presidential debates.
In short: I answer 'yes' to my own question, but 'no' to the greater question of whether he'll actually win the nomination.