Again this wouldn't even happen if "AUTOFILL" Wasn't an option
But if Autofill wasn't an option, there'd be longer queue times. Nobody wants that.
Instead of Forcing players you could add an incentive for supports or w/e role is lacking players
First of all, Autofill is in and of itself an incentive. "Play Support/Fill and you won't have to worry about Autofill for a little while."
Second, generally speaking, incentives wouldn't help at all. If people are already throwing shitfits and trolling because "i didn't wanna play Support" - d'you really think they'll play it straight for any other incentive?
[Insert Suggestions here]
All of your suggestions are poised to break a very meticulously moderated economy - Especially the RP reward.
Hextech Keys / Hextech Chests - Hextech Crafting rewards have to be particularly balanced around both of the items current acquisition methods. Key Fragments are radiant rewards from Honor (as well as periodic milestone rewards), meaning your ability to unlock Hextech Chests comes at a steady drip so long as you play by the rules.
And Hextech Chests - at present - are considered as a reward for skillful play. Scoring S ranks on a variety of Champions makes Hextech Chests sparsely earned, and (somewhat frequently) rewarding. Especially considering that, in the course of a month, you can only receive about 4~ Hextech Chests.
And since you don't say anything otherwise, I can only assume that your examples have no limits as to how many times you can earn them, which...Frankly, is a very dangerous proposal given that the more FREE Hextech Loot players are getting means there's less actual money Riot may be earning through Skin/Hextech purchases, meaning they have less money overall to spend on working on the game.
The same especially goes for Riot Points. 5RP per filled/Support match is not even close to a good idea, and would see very, very quick abuse by people who genuinely don't care to play Support or fill.
Again, I'm assuming that there is no limiter on the reward, since you neglected to mention any. A player could easily put in enough time to have various tiers of skins within days, weeks or months, effectively being able to get high cost skins without even considering spending money on it - without any moderation from Hextech Loot or anything similar.
It'd take roughly 90~ matches to make 450RP, 144~ matches to make 720RP, 195~ for 975RP, 270~ for 1350RP, 364~ for 1820RP, and 650~ filled games for 3250RP.
And, generally speaking, the incentive of "play this role that nobody likes for RP" presents what ultimately boils down to an incentive that screams "GRIND FOR SKINS".
That's not a good incentive. At all.
(granted, it may be nice for those who for one reason or another are 25~ RP shy of a skin they'd really want, but, still. Using Premium currency as an incentive is not good.)
For the Hextech Gemstones...That's honestly a little better than the rest, but it still runs some risks. For one, 100 matches for one Gemstone seems a little excessive, and (coupled with the fact that you need 5 Gems for a ward, 10 for one skin) is like the suggestion with RP, screaming "grind matches you don't really want to play to get free skins".
And, that may be all well and good for people who might be at 8-9/10, but in general, giving people long, drawn-out rewards that don't give immediate impact and demand you suffer through hundreds upon hundreds of matches that, for the very purpose being incentivized, you don't find fun...It's not a good idea.
Riot is being lazy about the whole Balacing & Fairness Aspect making people AUTOFILL is intentional GAMEPLAY SABOTAGE
Making people autofill is as much intentional sabotage as playing that sickass new Champion that got updated/released recently.
Autofill has more benefits than just shortening queue times - it makes players leave their comfort zone, whether they're normally the roving Jungler or the "my lane is the only lane that matters" toplaner, having people try new shit - for better or for worse - makes people learn the game better.
You may think it's lazy, but I think it's even lazier to ask Riot to incorporate more and more ways to get free shit...Just to make an understated role look prettier.