The game I think that you are talking about is Smite. Whether it is or not I made a post about this in a similar thread a while ago which illustrates why this would not be likely to happen. I'll just throw it below.
First off I want to say of course I would like additional skins for champions I play, but I am not sure it would work. While this system works in Smite it does have some trouble being ported to League.
If I recall correctly there are about sixty seven gods in Smite and this system have been in since launch. I did not play then but I think there were about seventeen gods at launch. This means at maximum Smite had to develop about seventeen 'mastery' skins and design them during a time there was no player base waiting for content.
Again if I recall correctly there are 126 champions in League. Assuming that Riot starts right now they will need to design 126 skins, essentially concurrently, to catch up with all their champions. Assuming they immediately diverted all the art assets needed to only ship mastery skins with Riots current skin release level we can assume about 3.5 skins per two week.
This mean it would take about seventy two weeks or about eighteen months with no other skins. Because skins are Riots only reliable form of income this of course would never happen. But with projected releases already at about eighteen months, lets say they only use about 1/4 of their art team. At that kind of pace this would take just over six years to complete.
All this totally bypasses the question of if Riot would even give skins for free, but with a timetable like this I highly doubt that this is something we would ever see.