Here are some of my proposals that would make donator ranks be worth donating for- without breaking the game:
Normal Donator
1x complimentary daily slayer task skip (reset every 24hrs)
5% static boosted exp rate for skilling
5% boosted slayer key drop rate
Super Donator
2x complimentary daily slayer task skips (reset every 24hrs)
10% static boosted exp rate for skilling
7.5% boosted slayer key drop rate
New Super Donator Cape - same stats