Based on Mastery Level (the numbers not the rank) you should give people the option of selecting rare Summoner Icons for that champion. Either use existing art, old skins, previous portraits, or some of the stuff on the website.
e.g. I've played this champ up to:
50k Mastery - Chroma Icon
100k Mastery - Holiday Themed Champ Icon
150k Mastery - Old/Alternative Champ Icon
200k Mastery - Specially designed Champ Icon as a reward for this level
500k Mastery - Icon that represents the champ in some way (Bard = A Meep; Darius = Bloody Axe; Garen = shrubs; Malzahar = voidling)