Bots should all default to the AI level of the bot game
I have seen this happen twice with two players leaving the game at Intermediate AI level and being replaced by Beginner level bots that feed the opposing AIs without any intelligence. This makes for a hugely long and painful game for the humans, unless they decide to surrender, because the stupid AIs will attack heroes under turrets, or attack turrets without minions nearby, and die, and die, and die; I have seen as many as 26 deaths (and reported it). It would take roughly two lines of code for bot players that replace dropped humans to default to the AI level of the match, which at least would keep it somewhat balanced.