Black Mesa's version of the long jump module would be perfect as a recovery move, tbh.
Regarding stages, seeing Black Mesa's take on Xen as or part of the Gordon's stage would be awesome. It still has the alien feel of the original, as well as severa lof the aesthetics, but they expanded upon and added so much to it that it feels more like a "real" world, and an alive one at that, compared to the original's Xen. Again, it'd just make the theoretical Black Mesa representation all the better.
On that note, I never actually disliked the original Half-Life's Xen levels like many do, but both visually and gameplay-wise, Black Mesa's Xen is far superior. It was rushed in the original, after all.