Why does there need to be a new rank? The forum doesn't need any money, it's not using the money that has already been donated.
The only thing this would do is make you feel cool because you had something different next to your name. That does not seem like a good enough reason to add a new rank.
A new rank would give people the opportunity to show their commitment to the community as represented by the crowd on these forums.
If that is a good thing or not...
It would certainly also offer opportunities to scammers.
My humble opinion: if there's demand, let them have their 1-BTC-rank.