The plan is to use milestones in the issue tracker for this as we go, but it's not finely detailed at the moment.
The big ticket items that I consider mandatory for 1.0 are:
1. Websocket support (and possibly a WAMP impl)
2. Reference implementation with an actor system (Probably Spring's Reactor)
3. Real time metrics / monitoriing
4. Reference integration with an Sql and NoSql store
I plan to cut version 0.9.0 very soon, then frequently bump until we get to 1.0.
API changes may (probably will) happen right up until 1.0.
A 0.9 cut is probably all I was looking for (possibly with the metrics improvements), currently Ratpack is working perfectly for our use case but the risk of building from snapshots was a worry, given that API changes are expected.