Get players to buy champions by introducing single player story/campaign mode

Create a single player story/campaign mode, with a different story/campaign for each champion. Players only unlock a campaign when they own the champion (free champion rotations don't count).

The first level of the campaign comes with the champion, but for the popular champions that many people already own, you can have additional levels which need the player to own other less popular champions to unlock (perhaps the other champion appears in the story).

You don't have to release it for all champions at once, just one at a time gradually as you build them out. I admit I have never spent a single cent in the game because it doesn't really change my gameplay, but with this I would have a reason to spend.

This can also pull in a whole new cohort of players who are interested in single player/stories but not that interested in PvP games.