Bonus IP won't help your Support problem

There are a few things that can do to increase support numbers in league, none of which include offering more rewards. People dont play league to generate IP. People play league to express their skill and have fun.

If you want more people selecting support, you need to increase both of those areas: Skill expression, and fun factor.

First off, in group play, Support can be very rewarding and fun. You have the means to set your team up for success and can cover metrics (like vision control, disruption, etc.) that other roles just cant.

Outside of group play though, the Support experience is a crap shoot because your impact potential is intertwined with a total stranger. Most typical Support picks can't just give up on the marksman and try to carry on their own.

In solo queue, every game has additional risk for Supports that other roles arent nearly as impacted by. Is the ADC the same skill level as I am? Is he going to be a jerk to me if he doesnt like my playstyle? Will he squander a lead? Will he rage or tilt if we give up first blood?

When you are a Support, it can often feel like you are at the mercy of the solo queue gods. Sometimes it's good, and sometimes it's a nightmare. Either way, the "feast or famine" play experience that is dependent on randoms, is not one that many players enjoy.

In regards to skill expression, Support doesnt have nearly as many champions with high skill ceilings as other roles. Outside of Support, you have numerous champs that specialize in skill expression like Gangplank, Yasuo, Lee Sin, Zed, Fiora, Riven, etc. Support has Thresh, and some could argue Bard, as high skill ceiling champs.

Outside of Thresh and Bard, its difficult for Supports to create noticable, high-skill plays that display how mechanically proficient they are with their champion.

If you want more people choosing the support role, you need to create more support champions that are flashy with a higher skill ceiling that feel unique in what they bring to a team. Right now they all really just feel bland.