It would be awesome if the jax-rs client implementation would be reworked to use more efficient http client under the hood. Jetty Http Client would be a good option, but certainly the upcoming release of the http client in java 9 should be implemented as well.

Ideally with a strong focus on http2

This should be the default client for cases where it can be. There are some edge cases with the javax.net.ssl.HostnameVerifier is set on the ClientBuilder.hostnameVerifier(). For these cases the Apache HTTP Client will need to be used. Per https://bugs.openjdk.org/browse/JDK-8213309 there is only a way to fully disable the hostname verification and a HotnameVerifier cannot be set on the client.