Microsoft's latest Windows 10 19H1 test build, No. 18323, doesn't include many new features that weren't in previous test builds. But it's one of the builds that is paving the way for this release -- likely to be designated 1903 -- to start rolling out to mainstream users in April 2019.
The new features in Build 18323 are incremental. There's improved RAW image format support. There are some tweaks and fixes to the new Light Theme coming to Windows 10. And there's a laundry list of fixes and known issues that are part of 18323, which Microsoft started rolling out late in the day (ET in U.S.) on January 23 to Fast Ring testers.
Microsoft is in bug-bash phase with Windows 10 19H1. If all goes as planned, Microsoft should "RTM" this feature update in March (hence the 1903 designation) and start rolling it out in April.
Windows 10 users who want a list of what's new for business in Windows 10 19H1 test builds should check out this Microsoft documentation page, which itemizes previously released test features like WSL tooling improvements; an updated Windows Hello PIN interface; the new Windows Sandbox and more. Just remember: There's no guarantee that any of these features will be in the final April 2019 update, however.
A related aside: Microsoft also has released a few test builds of the server complement of Windows 10 1903, aka Windows Server 1903 (the Semi-Annual Channel release of Windows Server). The new feature list for Server has been especially light and seemingly very incremental -- which may be intentional. I've asked Microsoft if there are any major new features coming to Windows Server 1903. No word back.
Windows 10 October 2018 Update: The new features that matter most