Interesting idea and understandable concept straight from the beginning. Also ambitious design - powerups, 3d-graphics, different forms for units/player creature, animated abilities, enemy weapons and shields etc.
Best thing were the actual core; infecting enemies to convert them to be in your side - even if there was a mass of them eventually start weaken and defeated.
Probably halving the LMB ability uses or increasing the power of infectivity would be nice. Also weapon range were not quite balanced - almost using drop range while enemies fire bullets from other side of the map.
Hijack concept idea wise up to me later; the creature player controls is able to combine with infected human soldier after a while from the infection/conversion was deployed. This hijack/capturing is also a clever and interesting detail in the overall concept.
Would be nice if characters who's converted shape different a bit; blood, acid, drip, etc. in their appearance. Also if enemies who are ready to be hijacked could be also turner into something else, like exploded into swarm of small alien spiders to distract enemy units.
Player could be able to spry some sort mud/ooze on the ground that enemies would try to avoid to step on, and by that able to infect approaching enemies but also delimiting combat lines increasing. Enemies could also patrol when player is not in sight, and display aiming before shooting.