We've had some discussion on the fact that aliens currently don't have as fleshed out a unit roster as the human factions(about 3-4 units fewer iirc).
Aliens are also missing something else to build from the grand cyst, since the Goliath solely occupies this cyst, and right now the Goliath is in a poor place balance wise where it's somewhat useful if you rush it on smaller maps, and then borderline useless for the rest of the game, due to the wealth of options humans have for taking them out past the 15-20 minute mark.
Aliens are also in desperate need of something to cover their mid-long-range deficiency since both Behemoths and Scorpions are fairly ineffective at their already fairly short max ranges when up against human vehicles with 3-4x their range.
My proposal would be for a hybrid artillery/tank buster unit, built from the grand cyst, perhaps costing in the 3-4k biotics range.
A slowish unit, which can't be rushed in to siege human bases particularly effectively, but if pushing with a combined arms(legs, tentacles) force, could serve to provide covering spread indirect fire as well as precision high penetration shots.
A unit which can shoot a spine(similar to the old Scorpion) only much much larger, essentially a similar size to a tank shell. Unlike the old Scorpion/Impaler this would have a decent cool down(maybe around 10-15 seconds), and a projectile speed similar to the humans HAC/Combat tank main gun, though with considerably higher projectile weight/drop(maybe closer to the siege tank). This would allow for higher skill cap on usage and prevent the same issue with the old projectiles mentioned, since you can't just rapidly ping spines off human units.