Make a clone and change the mining algorithm...oh wait...lol 
Ripple had most opposition among current bitcoin holders. If somebody would make a clone, where ripple ledger would be created by importing latest bitcoin ledger as genesis "block" (no history, just address-balance), all bitcoin holders would automatically become ripple-clone stakeholders. Additionally, exchanges with shared wallets would have a great incentive to become gateways, which are a critical part of ripple infrastructure.
and logins would be done how? hashed private keys? against addresses
Since bitcoin addresses are not compatible with ripple accounts, additional transaction type would be required: sweep-initial-funds. When user would first open a client to create ripple account (along with user & password to access it), he would have an option to sweep initial funds (multiple addresses) into his account. Other than that, everything would be pretty much as it is.