I'd say a way to make nests, which freespawn stupid(wandering) units.
Queens that you send somewhere, for it to turn into more of said nests.
Maybe 'Queens' or 'Nests' could be recruitment centres as well.
A bit like Kobolds, but in reverse, with controlling of stronger units manually, and weaker (but vast numbers) being uncontrollable??

Perhaps altering the world map to transform it into more 'alien' like biomes?
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Possibly summoning other alien types as "immobile" defenders, or as stupid wandering units.
A "Central" hive, which can be upgraded with gems (or other resources).
Central Hive is a "one of a kind" (like baron's baron, or barbarian leader). If it dies, it's re-recruit-able.
With it able to remotely summon new hives? Said hives producing freespawn at a distance, allowing them to overwhelm the world in crappy freespawn.
Hives could have an attack to summon a unit in combat each combat turn as well as self defence.
Anything none-freespawn for them would be manually recruited or summoned then.
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I think Aliens could have lesser tunneling capabilities than dwarfs, but with the advantage of being able to make 'air pockets', resistant against flooding, or the dwarf drills could end up nerfed to not be amphibious.
Tunneling would still be essential to them however, in the form of their 'freespawn' centers, being inside of tunnels.
Their starting position could be in Agartha.
A special "Nesting" terrain, which acts like the Dryads sacred groves, disappearing when not under the control of a Glyphid player over time, allows for freespawns to appear there, but can only be placed within Agartha, and a certain distance away from one an other (Like 5-6 tiles).
"Improved" Nests could be stronger, harder to kill, but also end up dropping "ores" for dwarfs (Iron) on kill, making it a risk-reward if you want to connect your tunnels, or place them in strategically unsafe positions.
Their tunnelers could have 'slow' (Big bugs to tunnel, but big bugs move slower, not as efficient as the DRG dwarfs)
Rather than an intricate system of tunnels Dwarfs might make, it'd be more like holes on the surface leading to small Agartha bases generating freespawn which come out to harass the surface.
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Stationary summons could be stealthy, with it being DRG Fauna and Flora that arn't Glyphids. Like lootbugs which improve a locations resources, grow bigger, but give resources on death to opponents.