I have a suggestion of content to be added after the Xen levels are complete:
Make some levels that connect the Half-Life 1 ending (eg Black Mesa Source Ending) seamlessly with the begin of the Half Life 2 beginning. (like the episodic content does link seamlessly with each other)
I dont really know or understand the story completely so I dont know if its possible “story-wise” to do that, but to my understanding, the HL1 story is in some way related to the HL2 story.