I'd like to offer a solution to the current problems with the -fill- choice.

Maybe adding IP bonuses for those who choose fill. 25% for People who choose it as their main, 15% as secondary choosers. This MAY entice more players to choose these roles.

Another game I play, added an option like this 5 years ago, World of Warcraft. All classes on WoW have a DPS spec, and it is the easiest thing to do when raiding since it is the least gear requiring, and the easiest to get carried in. Tanks must know the mechanics of the fights, and healers have to keep everyone alive regardless of everyone else's knowledge. There tends to only be 2 Tanks per group, 3-6 healers, and the rest DPS. The problem WoW had was that there was an excess of DPS characters, and not enough tanks/healers. You would end up having a queue for a raid an hour long, waiting on 2 tanks or a couple of healers, but have all the DPS needed. WoW ended up adding an item given to players who chose the most needed role at the time. (The role that would make the longest queue's start). This is the current item for queueing as a high priority role This gave a chance for players to get older gear and mounts by playing as a role the queue needed, so that the queue would start quicker, and the person playing the (maybe) unwanted role would still get a reward.

Maybe add an IP bonus for the fill option, incorporate rewards to correspond with the new Hextech crafting system.

Sorry if my idea is incomplete or isn't clear enough for people who have never played WoW, I'm terrible at writing.