I was thinking - since most basic enemies have some sort of bigger or more dangerous version that appears later on in the game, the Mantis could use some love? Some sort of massive Mantis that guards cave entrances and quickly dashes at your submarine before weaving out of the way would be cool. Maybe it could even attach itself or grab your sub to drag you downwards, since the Mantis is the only enemy in the game that actually has appendages that look like they could be used to grab onto something. If killed, its drop could be processed into an extremely potent paralysant, or something
Overall I just think some more enemy variety for the lategame would be great, since by the end it felt like I was seeing the same things over and over with not much new to really take in