The AFK Solution
A few years back cartoon network released an adventure time themed moba that was very similar to Twisted Treeline. The game was pretty simple, without any items or summoner spells, or a large character pool, but it did address the AFK problem significantly well. I noticed this when in a game against a player who had lost both their teammates. The player was able to three v. one with no problem. I then looked at the players buff bar and noticed a buff for when they had no teammates.
The answer is dragon. The dragon buff is a stacking buff that goes from being negligible at the first stack, to being a win condition by the fifth. By implementing a similar buff to a team who has lost a member, the playing field is evened out. The AFK buff however, should be on a 3-5 minute timer after the AFK has left to avoid AFKing while dead to help out your team.
Buff ideas:
1st AFK: defensive buff to survive laning 2nd AFK: buff to damage (greater against minions) 3rd AFK: tenacity to avoid CC chains 4th AFK: game ends immediately