I feel the pain! (Supp main) Sadly i don't think they can change it much without setting the other roles out of proportion. I agree that it is insanely difficult to get an S-, S, and S+ as a support. I have been trying to get my thresh mastery 7 tokens for the past couple weeks and still have none. But you have to remember, assists aren't everything. The ranking system still take your cs into account, so if you are playing a Face of The Mountain champion, then try and focus on getting cs as soon as the proc is up. I know this won't help much but its something to consider. :)

Another option is roam a heck of a lot more to get even more assists, but that is only if your adc can handle themself while your gone. There is no set way (i don't think) to determine who is the "Support" in the game, so they can't give you a better grade based on the assists, etc. that you get. So as of now, all you can do is get more and more assists. :P