Completionist cape would give maxed players something else to grind after already maxing and it would be a cape with better stats then maxed infernal, god cape and ava max cape. The cape could be achieved by being maxed and completing many boss kc at each different boss/raid, all quest completions, tasks and diarys.
Below is some rough required achievements in order to aquire the cape
Boss KC
250 Abyssal Sire
250 Alchemical Hydra
50 Barrows
250 Cerb
100 Chambers Of Xeric
100 Theatre Of Blood
150 Corp
250 Dags
250 Giant Mole
250 Gwd bosses
250 KQ
250 KBD
250 Kraken
250 Thermonuclear Smoke Devil
150 Gauntlet
150 Vorkath
150 Zulrah
10 Jad boss
10 Tournaments played
Tasks
500million Total XP
Voted 30 times
All quest completions
All diarys completed
Bunch of skilling requirments E.g chop 100 magic trees etc etc
99 All skills
Feel like this would give players more to do/grind because after maxing there isn't really much else to do apart from farm gp etc because the current collection log isn't one that really looks like worth grinding out unless its updated similar to osrs.