In their latest post the HHVM project (of Facebook) has laid out the next six months ahead for the development and progression on the project. In it they talk some about their "themes" and overall Open Source goals planned for the first part of 2014.
The HHVM team has just wrapped up its planning for the first half of 2014. We'd like to share our plans, providing you a bit of context. We've been making steady progress on HHVM's compatibility with PHP in the wild, but we still have a lot of work ahead of us. We're using unit test pass rates as a proxy for success measurement, but you can help by adding HHVM to your Travis configuration, and reporting bugs and issues through GitHub. We are resourced to help support a couple of major HHVM deployments, which we hope has the side effect of exposing us to "non-Facebook" deployment and maintenance challenges.
We are also going to push for a more open development model, with the goal of increasing our community participation. We'll have more to say on what this means later on. Stay tuned!
They also cover some of the work being done to increase the overall efficiency, reducing CPU time and memory consumption. There's also mention of work being done on a guide to "hacking" in the HHVM, reducing some complexity in the compiler and the conversion to a full HNI extension interface.