I think it just comes down to 1. is it a viable skin concept (hasnt been done before/isnt too similar to existing skin(s), and is feasible from a design perspective)
and 2. is it something a lot of players will like (Riot's a business after all; gotta make investments worth it)
Player skin concepts don't get made into skins very frequently but it certainly does happen.
Take for example my personal favorite skin Heartseeker Varus-
deviantArt user SuperKusoKao designed it, I assume just for fun, and someone at Riot liked it enough to make it happen. The design changed (legal reasons maybe? I'm not sure) but the concept is still there.
IIRC, Battle Bunny Riven was also originally a fan concept.
Put your idea out there, try to get some support from your fellow players, and it just might happen. Never know if you don't try :)