I could assist as well, but the scope would have to be well defined. Are you simply looking for lore references so that you could just put a D&D game in valoran? Are looking to create a campaign setting based off of Valoran and its themes with custom classes, items, races (yordles)? Are you trying to make a d20 game system which will make you feel like one of the champs from league (new classes, abilities, power systems)? What version of D&D would you want to be basing this system off of?
Not trying to overload you, just hoping to provide some direction so it could accurately be achieved.
Hm, I had some ideas plotting where say.. Instead of creating a type of class that every champion would under their own.. I decided that maybe (as an idea) perhaps they could use the whole "primary-secondary" thing champions have in-game. Such as, assassins could get this trait, as tanks would get this. Pretty much all 'type' determined in Team Builder. Fighter, Mage, Assassin, Support, Tank.
Like, maybe the player would decipher what their primary role is and secondary. There would be advantages to choose only a primary (with no secondary) as the type determines the general idea of ability type capable of using. Also capable of customizing the idea of their character. Each canon race would have it's traits, etc. But if you are building a human, Mage-Assassin (just a random example). You would benefit from some AP usage (and yes, an AD, AP system). But say you use some type of shamanistic surge of..idk..earth.. at the enemy. We will have stats available for something of such nature that the DM would primarily detemine such usage.
and no, lore is not a problem with me. I love LoL lore and I can deal with it really.