Most likely to be in a lore event vs least likely

I would say that Yasuo would be the most likely. He has an interesting story, is one of the most played champions and lends himself to have interesting interactions with other characters. Even in future videos he would be an ideal protagonist as his fighting style is interesting and exciting.

Rammus is obviously the least likely. He has no dialogue (except for ok), allegiances, history, personality, rivalries or incentives. He is seldom seen in the game and his current lore makes it look like RIOT wrote it in roughly 43 seconds. His fighting style consists of rolling into a ball and letting the enemy whack away at him. You will see League of Legends 2 before you see a Rammus story event.