I can't see Riot going out to make a skin for each champion once someone reaches 1 million champion score with a single champ (hell, I only have maybe 300k combined) but I would like to see maybe a variant of a chroma come out for the base (classic) skin that would make them golden or shiny, or something else similar. Even something as simple as a fancier loading border (or overlay to the loading art, like how the PROJECT skins had) would be nice to see. At the very least, maybe a summoner icon of the champion reached 1M score with would suffice just using the mini-map art, although personally I think that's a bit of a easy way out and not fitting to a summoner who is clearly devoted to that champion.
The only issue I have with unlocking a mystery skin to a 1M score champion is what if that person already owns all the skins for that champion or don't like the others they haven't bought? There's a reason most people buy one skin over another, and that's personal preference. If I got the Deep Terror Thresh skin for example, while I do like it, I wouldn't use it over Championship or Blood Moon. It feels kinda pointless when you get a skin you don't like, so I think this reward could feel a little un-rewarding. Now this obviously won't occur to everybody, especially those champions who have cooler legacy skins they might get, but that is my initial thought to the idea of it.
Either way, I definitely feel that there should be something given to those devoted enough to a champion to get to one million score. I can't even play one champion to 100k, so to me 1M is an insane amount of work. Kudos to those that reach that milestone.
) it won't go easily unnoticed like a loading screen boarder and isen't so hard to make