I think that this has been suggested multiple times before (correct me if I am wrong) but honestly, even if it would be good from some perspectives, it would still be quite unfair and also weird to add after such a long time.
Some players on certain leaderboards have been there for so long, probably as long as I can remember and beating them would take quite a lot of time and effort. Getting on a leaderboard itself can be kinda hyped but rewards for this isn't anything I personally think should get added.
Having seasonal leaderboards with rewards or tournaments in different gamemodes would be a better option. This could be a thing with featured games, to add them for a week, have some sort of tournament with prizes ect.