Idea: Give players who end up playing as a support role, extra IP for their troubles.

So... Riot. You're trying to get people to play support, huh? But so far most of the things you've tried haven't worked, have they? Well perhaps try this, give players that end up playing as a support, extra IP for their troubles. Let's say.... 50% boost to IP for playing support perhaps. That'll surely get some players who are on the grind to consider playing support.