No, I don't care about graphics, so any improvements can be made with the current engine. I'd play Dota 1 on WC3 no problems if I enjoyed its gameplay more than League.
What you have to realise is that a video game engine is never rewritten. Throwing an entire engine out and rewriting it from scratch is something NO sane developer will do. Any developer that claims to have made a "brand new game engine" tends to be lying. Skyrim's Creation engine? Same Gamebryo engine from Oblivion, just with modifications. Dota 2's engine from Valve? Same Source engine used for HL2, with modifications. Source engine for HL2? Same GoldSource engine used for HL1, with modifications.
Developers only claim to make a new "modern" engine for marketing purposes. In reality, the engine is always just the same one from the previous game with upgrades. Since this is a rolling upgrade release game, Riot can make any necessary upgrades to the current engine (as they have been doing).
Hell, gameplay wise, just in the last 2 patches they made major changes to the way skillshots work. It's barely noticeable, but skillshots are now detected a lot more accurately (e.g. before you'd occasionally get Ashe arrows that hit way too easily or Lux ults that completely miss a target in the middle of it).