My suggestion is lvl 120 in skills, along with the corresponding skillcapes, aswell as max/completionist capes and their variants. It's not in OSRS, however, its sexy as hell, and I think a lot of players, myself included, would love to see them added. In addition to the cosmetic appeal of the capes, it also lengthens the experience of the players and gives them a higher achievement to work towards beyond filling their banks with riches.