One Skin, all champions.
Let me explain. A skin is pretty much new clothes, technically. What if I could put this outfit of clothes on any champion I own. Wouldn't that be cool? I think so.
I know how there is "Riot police" skins and "battlecast skins". But they are only limited to those champions. So creating a comp around those skins isn't exactly ideal. If there was "Riot police" clothes that I could put on any champion, having a Riot police team would would be so cool.
You could have gangster clothes and have a cops vs robbers game. Or super hero/villain clothes and have super hero vs villains game and so on and so forth. The possibilities are endless!
I like the idea of matching skins in game and wish you could do that with all champions. And in each of those scenarios any champion could play the role of cops and robbers theme or super hero theme.
And blah blah about the work behind the idea, take that crap elsewhere.
and 
or
not to mention that some champs don't have legs either like
or
or champs who can transform like
or
would also be problematic.