After having 80% leavers on my team during my first 5 ranked games, I began to consider a way to fix the situation after raging pretty hard.
The Ranked Rescue Queue!
Players can choose to enter this special queue that will allow them to jump into the shoes of a player who has left the game. When a person disconnects there is a 30-second delay prior to the game becoming viable for rescue queue. When a player enters a preexisting game using the ranked rescue queue they will take control of the champion who was left stranded. All masteries and runes for that champion will be maintained (even if the new player does not have such things).
The player who left will still receive a leave and a loss. The new player will enter the game but with a lower ELO gain/loss than normal (as they are volunteering to jump into an already bad situation). The intention of the queue is to replace the leaving player with someone of close ELO so the game will not be swayed in either direction. Also the queue will attempt to only give players control of a champion they own already.
Players can enter the queue from the typical join screen. While in the queue they will have the normal count-down. When a game is found it will notify them, and all other players of equal viability (ELO, and champion ownership) at the same time. The first player to accept will enter the game. The notification should list the orphaned champion they will be taking control of.
TL;DR: As the idea of bots replacing leavers is not viable, please consider replacing leavers with players.