So with new lore there is no justification for completely unrelated champions to fight.

SR confirmed non-cannon. Hell, all modes confirmed non cannon. There is the aram map but its position and place in the universe doesnt mean anything because you play every champion on it, with any combination. Make yourself a Sivir, Lucian, Tristana, Yi and Shyvana team fighting on Crystal Scar even though they have nothing in common because why not. Experience the best immersion league has to offer.

My point is, since League's champion pool is so big, there can be easily a mode that limits your pick choice to a specific faction (ex full Demacia team) and puts you against your opponent (Noxus team). Or depending on story's progress (if there are any plans for one), create teams of 2, 3,4,5 players limited to picking certain champions. Of course, it would have to be an all pick game type since not everyone owns that Nasus or Cass.

As of now, its perfectly fine if there is lore being remade but if it does not aim to improve player's immersion and make him feel lke he actually participates in something big, then I'd rather just go and read some fanfics. Just about as relevant to the game I play.