Would this (eventually) include the unreliable datagram extension?

Don't know if it could get faster than UDP if it is on top of it.

The use case for this would be running a multiplayer game server over QUIC

Or a mix of both, like datagrams for voice chat / movement and reliable packets for important data

Other use cases include video / audio streaming, VPNs over QUIC, and QUIC-over-QUIC (you never know)