Develop a design to support multiple bridge authorities
The main thing blocking multiple bridge directory authorities right now is that we don't have a design for how it would work. For the normal directory authority design, we want all of them to know about all relays. But for bridge authorities, that would defeat the purpose. So we want some algorithms for distributing bridges over authorities, such that bridge users know where to go to look up a given bridge (probably as a function of its identity fingerprint). Perhaps the algorithm should provide stable answers even as we change the set of bridge authorities, and for clients and bridges running a variety of Tor versions. More generally, we need to figure out what functionality we want and what security properties we should shoot for.
Somebody should start with a proposal, and go from there.