Simple really, once you hit certain landmark exp totals in a skill, add an ability to the cape.
at 50m exp, ability to change colour of trim.
at 75m exp, ability to change background color of cape.
at 100m, +10% to cape buff.
at 125m, discount on supplies from vendor (feathers/arrows/runes)
at 150m, personal notes where possible?
At 200m exp, ability to obtain cape for free from vendor (no gp for alching)
etc.
Ideas ?