The last build released for mobile was build 10166 and it brought some much needed polish, including some new animations and bug fixes. This build also made it to the slow ring a couple weeks later. According to Aul's tweet, the next build will be greater than 10240, as build 10240 had a bug which kept it from making it through the necessary internal rings. Build number 10240 is notable as it was also the number for the RTM build of Windows 10 for desktop selected for release.
We haven't seen many leaks regarding builds after 10166, but we should expect a more stable build, with few new additions as it is nearly feature complete. It also wouldn't be a surprise if the new build included a near complete version of Continuum for phones, recently teased by Joe Belfiore in the tweet above, but as it requires special new hardware, only Microsoft would be able to confirm this.
Gabe Aul has said they are in the process of moving the mobile build to a new branch, which takes some preparation time. Recalling the desktop branch shift from fbl_awesome to fbl_impressive, moving to a new branch usually represents a significant milestone.