So when it comes to League lore, they do (or recently have been) doing a great job with it, and reworking it. However, there are ways it could be further expounded on, so people in the Skins & Champions Concepts forum branch or even the creators of the game could expound further.
They could add another continent thats roughly the size of Valoran, or just barely smaller. One reason this could be good and beneficial is that players on the forums have been complaining about the lack of black or just all around toned skin champions. If you created a continent influenced more by Indian culture and African tribes you could easily situate more racial diversity into your game, and like what you did with Rek'Sai and Gnar give champions interesting lines, or create a new language that people can only guess at.
Scratch out things on Runeterra that don't nor have seemed to play a role. They are taking up space and with no leaks or rumors yet floating around about champions coming from them (ie: Anything in the upper left chunk of Valoran's map) or at least give them purpose/create champions from there. I mean the Voodoo lands is a WASTELAND something could definitely come from there, but has it? Nope (at least not to my knowledge).
The seas around have been very unexplored, Nami and Fizz are the only TRUE sea dwelling champions, and half the map is water....soooo....
That's all I can think about for now, if other people have suggestions feel free to comment!