What's stopping someone from forking Ripple and running a server where the transaction fee tokens are backed with Bitcoins or USD?
The currency used to pay the fee has to be built-in to the system. You can't use the blockchain, because then every Ripple transaction would have to wait for confirmations. You can't use USD because those are only IOUs in the system. Also, Ripple destroys the transaction fee. That would mean money being wasted!
XRP solves an impressive number of problems in the Ripple system so you can't really do away with it. More practically, a fork could eliminate the genesis ledger which puts 100 billion into a predetermined account and instead allow XRP to be created through proof of work. This mining wouldn't be necessary to secure the ledger (this is already implemented using the consensus process).
