In short: we are working on it.
Migrating the code from Katana to ASP.NET Core was actually mostly mechanical. But obviously new approaches and patterns have been introduced which might, or might not align directly with how we used to do things in IdentityServer3.
We also wanted to take the time to do some re-work and re-thinking, as well as doing some breaking changes that we couldn’t easily do before.
For a roadmap – in essence we will release a beta including the new UI interaction next week. Then we will have an RC by August and an RTM before the final ASP.NET/.NET Core tooling ships later this year.
Meanwhile we encourage you to try the current bits and give us feedback. The more the better.