I think many would love a game mode like this

The issue with lol is that the game has too many champions. It's impossible to balance if you don't have enough employees to do the job.

A 5v5 restricted pool game mode.

You would only have champions until jinx release. The reworked ones would get reverted.

The monsters that riot created were all done after the jinx release. Before that the champion pool was less oppressive and less numerous.