Solution for ragequit and afk that never return to the match--mid game replacement queue
There should be a separate mid game queue to take over a champion that has AFKed. The win and loss of LP would have to be adjusted probably for this player, but basically, if you have the same champion (or, maybe even if you don't?), while you are in queue, a game will pop up and give you a good amount of time to decide, like 30 seconds. It will tell you how long the player who's role you are taking over has been AFK, how far into the match the game is, the levels of all champions and the score, drakes and turrets etc. You would have to play the champion they chose, with the runes they choose and summoner spells.
By the very nature of this, you would usually be a disadvantage if you are taking over a role where it has been 4v5 for say, 5-10 minutes. Therefore, there should be a reduction in LP lost if you lose the match, but still be able to win the same LP. This makes it a risk that a player can choose to take.
This serves a big function: All is not lost when a teammate AFKS! I would say that the option to fill the role should occur 5 minutes after a player AFKs. If they try to come back after that, they can't and they take a loss.
The only downside to this is that players might AFK MORE, because now they don't feel bad that they are bailing on their team...but I think most players in ranked generally don't want to take the hit for an AFK