I suddenly just wanted to draw this. I'm kind of sad the new games didn't reference the old much visually. The appearance of this armour varies but overall it sticks to the same look. I gave mine some additional padding/seams/panel lines. The original style is more of a skintight superhero suit I guess.
If I were given millions of buckaroos to do an X-COM game, maybe I'd go for a functional lowpoly 3D style to keep the pixel look but save on the effort of doing rotations which just seems like a pain to either pixel or draw.
EDIT: Chryssalid, Snakeman, Celatid!
The Chryssalid autopsy and iso sprite differ in colour, so I chose something inbetween. I made the Celatid more... anatomical than just a flesh sack. The proportions for all these are more on the realistic side... maybe inventory portraits. The original iso sprites might be something like 4.5 heads.
EDIT 2: Variants, Silacoid, Ethereal
Might be neat to expand with variants rather than new aliens, to keep X-COMs identity. One thing which sometimes bother me in games is enemies keeping exact pace with character progression, kind of erasing it. It could be addressed by sometimes including weaker variants, and sometimes increase difficulty with enemy numbers rather than some kind of power matching/escalation.
The Silacoid might be based on the Star Trek Horta. In X-COM it appears to be a rock creature with a molten core. Some kind of slug, I'm thinking. It's kind of fantastic how a worm can become a butterfly via chrysalis. The Chryssalids seem to have a similar morphing power... maybe a bit too rapid, for gameplay reasons. I'm not sure if it commonly comes into play though. I was thinking they should probably have intermediary pupa stages. Late game you're more likely to encounter more developed ones. IIRC the 40K Genestealers have like 3-4 stages, going from human to full alien melee monster.