Several Things Riot Could Sell

1. Different Voice Actors (Such as Bilgewater)
2. Champion Select Skins
3. In-Game HUD Display Skins
4. Summoner icons with special effects; for example, if you press F7 you could have a Poro icon pop up on top of your head, similar to Mastery Badges and the "They are Coming" sign.

Add more ideas below in the comments.