There should be some sort of incentive for reaching max XP in-game, as it can be a very difficult challenge to achieve & at this moment we only have a forum award that showcases when a user has reach 4.6b xp. I'm all for it, just not a huge fan of the cape you have showcased in this suggestion.
Would love to see some sort of other incentive(s) implemented - maybe the possibility of adding particle effects to max cape? But can only be unlocked once reaching the total max xp. I normally wouldn't suggest Rs3 content, but given the context I decided too.