Black Mesa: Xen is coming out soon, if its latest beta test phase is anything to go by. Developers have pushed all of Xen and Gonarch's Lair maps to the beta branch, which means there's plenty more for us to run through.
This is the second stage of the game's drawn-out testing phase, with the first one having only featured 3 maps, while the Xen/Gonarch beta brings this number up to 9 dedicated levels.
The final portion of Black Mesa has been awaiting the expansion of its beta testing branch for some time now, though the developers have clearly been making leaps over the past few months, with how comparatively fast they're now pushing out content to the beta.
Sometime in the future, beta testers should also expect for the Interloper, Nihilanth, and Endgame chapters to release for testing, and this will essentially double the number of playable levels: from 9 to a staggering 18, which is an impressive expansion of Half-Life's source material.
Some issues are already known, with the most prominent being that beta testers must use fresh savegames due to compatibility concerns If you've bought Black Mesa before,you can enable the Xen testing branch by using Steam's Beta feature, in the game's 'Properties' menu.