Would you donate if those ranks were implemented? What benefits would they have? I can't image many people will donate money for new ranks if they ever get implemented unless there's some special perks.
That's the point...making perks.
Perhaps for highest rank, implement a "trusted marketplace" of some sort, ability to leave a +5 trust as opposed to +1...the ability to leave -5 rep as opposed to simple -1 rep and so forth. There are many features that can be implemented with ease.
You shouldn't be able to buy privileges like that imo. I also think donating even for ranks wont be very fruitful. Most people would rather just keep a hold of their coins, though theymos has stated in the past new ranks may be added, but I don't think they're a top priority for either the forum or users.