Hello this is my first post on the site so be gentle, also had a quick look and didn't see any similar recent forums.
So in Xcom there was a pretty fair amount of enemy diversity, but overall besides the three basic alien types (melee, ranged and psychic) you rarely needed to switch up tactics to repel a change in opponent. Given the trouble making different species it makes sense to make battling each enemy type feel very different. Here are a few ideas I blearily came up with while my mind wandered, im pretty certain you can all come up with better stuff.
The Carrion Reaper: Goes after corpses to inject larvae rather than living things. Gives the ability to stuff corpses into your backpack for relocation more weight and turns something so seemingly simple as a corpse into an enemy asset needed to be captured. When not engaging in legitimized necrophilia it'll attack humans as fast as a regular Reaper/Chrysallid yet perhaps not quite as deadly. Defending corpses would be paramount to avoid being overwhelmed, anybody you don't want endangered could be made to haul bodies from danger zones.
Invisible melee enemies: A Xenonaut is able to see it only when it becomes close, lets say 5 squares. This means hording action points for reaction fire and closely guarding eachother's back's is a much safer formation when you know one is lurking about. Combine this with a long ranged troop and the player must make careful decisions about positioning and line of sight. Looking down a street and seeing nothing might be reassuring, or worrying if you suspect these things are about.
Crawlers: Climbs up walls, or perhaps its an alien fungal that grows on walls. Either way shoots from above with sticky biological stuff causing a unit to spend more action points when moving for the rest of that battle.
Anyway its getting late please post more ideas for enemies! Oh and I also understand its probably too late in the development process to add anything big anyway but nevertheless...