Title explains all, but the advantages of this are endless. I know there are plenty of maxed players who want something more to "grind" for. This is an excellent option and will provide players with a reason to do so much more in game.
Whether it be getting full void/full graceful/pking/skilling/boss kill counts (time requirements too) etc. New updates can constantly be added as well over time in order to wear the cape.
This is one of the few customs I support, and I hope the rest of the community does too.
I understand the argument that osrs doesn't currently have the competitionist cape, however 90% of the community has already voted in favor for 200m exp capes.
The cape doesn't even need to have any added extra bonus' - it could even be just a visual update..
This would increase overall activeness and player retention. @Omicron