Long story short, they are expecting to officially release Mir 1.0 with an ABI guarantee early in the Ubuntu 17.10 development cycle. It's yet to be firmly decided whether there will be one major release before going to v1.0 (it would be v0.27), but it's looking like not too far after Ubuntu 17.04 is released next month, we could see Mir 1.0 declared.
Mir for Ubuntu 17.04 meanwhile is settling around v0.26. Besides going for a stable ABI with 1.0, they will also be removing deprecated APIs ahead of Mir 1.0.
Following Mir 1.0, Alan says it should clear the way for Mir support in Mesa and Vulkan. We've seen them working towards Vulkan support on Mir but so far it's incomplete. It will also be interesting to watch if upstream Mesa developers will accept Mir support to be merged into Mesa as previously they were against such patches.
The Mir 1.0 details were shared today via this blog post.