How will they decide which ones to include as playable? I can't imagine them making a fighting game with 140+ characters in it right off the bat.
And if it's anything like the Smash community, there will be so much salt no matter which ones get added from everyone whose favorite champ wasn't added.
They'll add all of the skin sellers for "reasons".
Yasuo Riven Kai'Sa Miss Fortune Jinx
You get the idea.
Then they'll add a few who are okay at selling skins and reasonably included in fighting games.
And the 5 I listed are pretty automatic inclusions. There's precedents for each character's "fighting style" in the Marvel vs Capcom series alone. Heck, Riot would probably copy a few of the super moves from that series just to put them on their characters (like Morrigan's missile bombardment hyper move being put on Kai'Sa).
Most likely they'll go with champions who would flow into a fighting game very well already - Yasuo, Lee, and Riven come to mind as top candidates. They'll probably go the route they're going with TFT, however - Start with a decent roster to get the game out and for playing, then slowly add in others as they make them.
Not all champions will be able to make it as playable characters, due to their abilities in League not even remotely resembling something that'd work in a fighting game (Soraka, as an example, would have to be nothing like Soraka in League to actually be playable, so she's probably not going to make it), but they might show up in the form of assist characters.
