When I was looking over the cut enemies the only 2 I thought had any real worth was.
Kingpin - but he is not really an enemy he is plot driven. But he is a bit of a missing link. Design is solid. Based off the Guild Navigator from David Lynch’s Dune. He may have had some sort of purpose to do with the xen teleports given what he is based off folding space and so on.
Stukabats - the absence of these actually hurts the gameplay, i would even go as far to say “on a rail” is pointless without this enemy. The design is solid. Based off monster Vampire Bats.
Flocking floaters - another cut flying enemy but I think they look rubbish design is also bad. I think what we can safely assume though is tbat not enough flying enemies. I would swap actually change this enemy to be HECU flying sentry drone with small underside gun. Similar to man-hack from Half life 2. Scary Tecnology.
Probably would ignore everything else apart from Human Sergant he would make a good HECU boss fight. Typical action one on one fight.
I think Black Mesa focusing on variants is a good way of handling the lack of different enemies to fight over quite a long game. Flying enemies is really what is missing from Half Life and certainly a plotless Xen.
Chumtoad is a weapon.