Hmm well I'm not on the champion team, but if I had to design the appearance and story for a thief I would start with a motive and a source of power. The only thief archetype that works as a heroic character would be Robin Hood style where they are stealing and redistributing power in rebellion against an imbalance. Believe it or not, stealing is by and large frowned upon so a villainous character might work?
In the game of League what could this champion be out to steal? Gold is probably off the table because that would just be a terrible experience for players. Items same as gold. Experience would be equally misery-inducing. Mana has been tried in the early days (and not everyone has it). Vision? Damage? Time? Stats? A new resource created just to be stolen perhaps? Thematically they could be trying to steal the very legends themselves - the memories or stories that made these champions so great... not sure how to convert that into a meaningful mechanic though.
So maybe they are trying to steal the memories of the champions? A mind thief could be a cool sort of mage and I'm sure there could be some mechanic around stealing their experiences and using it against them. Almost every time I try to come up with characters they end up being mind mages. I guess that's just my jam. I can see from some quick ideation though that anything meaningful that you could steal in a game would make it as bad an experience for your opponents as actually getting robbed, so I guess that could be a blocker. But I don't know this is just me taking a crack at it.
to name a few), but none of them have really kept it as a running occupation, or at least don't give off the "thief vibe".
??? her kit is kinda like a thief