Lore expansion
Riot is pretty good at developing interesting characters and fantastic at world building, especially considering the constraints of a moba. All you have is a couple playable maps, but throughout the years, the world of runeterra feels as tangible as something like Harry Potter or Lord of the Rings.
That said, many gaps are generally filled by the players. One person's Runeterra will inevitably be different from another's. I think Riot has laid a solid foundation to commission published works of fiction based on their current lore, fully canon. This would give them the opportunity to expand beyond the game and solidify the characters, tangential or otherwise. Their current artists would be brilliant at cover art and inserts and I'm sure finding lore writers or general authors that want to flex wouldn't be difficult.
Riot should seriously consider it. They already have an audience. And the books would serve as permanent advertisement for the game years down the line, and players who cease playing will have something new to explore.
The idea came about because I thought about how cool a story narrated by Aurelion Sol would be. The little bit we got on his release was satisfying, but many facets of his character could still be explored. He's playful, arrogant, judicial, snarky, vengeful, dedicated, and a little bit soft, though he'd probably never admit that.
Anyway. I'd purchase anything if it is immersive and true to content. Just don't pull a Galio again...lol