Just like Sarah Palin was?
Sarah Palin was given a national stage and became a leader as a result of her being on the ticket. She may not have been a leader within the hierarchy of the GOP, but she became a leader just the same.
Well, yes and no. Until she declined to run for President, Palin really was the effective leader of the GOP, at least from the point of view of both the rank-and-file and the White House: she resigned as Governor of Alaska to become just a private citizen, but the Obama White House rushed around responding to her every tweet, every Facebook post, as if she was actually the leader of the opposition. To use the British political meaning of "shadow" she was functioning as shadow POTUS.
Ryan would be left in the same position, but with years of direct Washington experience. Of course, we hope this is a pointless hypothetical, and he'll be chairing the Senate (when he wants to) and going to cabinet meetings the next four to eight years.