Ratcheting
Briar's forward secrecy is based on periodic key rotation rather than ratcheting because we need to ensure forward secrecy even if no communication occurs for a long period, or communication only occurs in one direction. However, we could also use ratcheting opportunistically, so that the exposure of a transport key doesn't expose all future transport keys (the reverse of forward secrecy).
It would make sense to have a separate ratchet for each transport so that the ratchets for low-latency transports can advance quickly, but the ratchet keys for each transport could be synced over any transport.