Start making threads asking people to buy the _existing_ skins for that champion. That's how you get new skins made. If people are gonna continue to buy Yasuo skins and not Vel'koz skins, then Riot's going to continue to make Yasuo skins. They're going to use the people they pay to make skins to make skins that will make the company money.
So if you can get people to show that skins for that champion make money, they'll do it. You're not going to get anywhere with just the backing of a less-popular champion's reddit community.