I've commented on a similar thread before. Yes, it would be incredible, and yes Riot has already made boss fights, and could do something. But the problem is, while the graphics of this game have improved by a ton in recent years, the engine is still the same. Everything has to be coded as minions. I think even Baron, Rift Herald, and Dragon are coded as champions, even if they do give a CS.
An idea I have, is a minion set up like a Villager, and some type of monster attacking a Village. (Wolves, Voidlings, it doesn't matter) and the players could be sent in from a pool of Demacian champs to protect the Village. Anyway, that's a basic concept of what could MAYBE be done, the hard part is making it difficult. Riot had an issue just making the bots even a tad difficult with this engine, I can only imagine how hard this would be.
TLDR: Maybe this is better as a different, spin-off game, of League. On a NEW engine. Maybe that could even be why they removing all of the limitations that the Institute of War made, in the lore, was a good thing.