Love this idea. I know I had an idea that is kinda like achievement diaries in rs.
So if all easy tasks under a category like general you can get a some sort of helmet and just like in rs as u complete each tier of tasks the rank increases from helmet 1 through to helmet 4 for all achievements done. So if general gives helmet, combat will give lets say a platebody, skilling platelegs and minigames some sort of shield. Also the achievement dairy cape being a reward for those that have completed every single achievement would be pog.
I've also thought that maybe just the cape but different tier of said cape so maybe the green on the achievement cape is turned to white. Then once all easy and mediums are complete the cape turns to red, then orange once all hards are complete then to the standard green for all achievements