Here's how I would have done the Nightcrawlers:
- Just a smidge more human; in PM they were kinda flat, and didn't really "react" per say to getting spooked, shot or killed. Make them drop their stoic mask once in a while, like the Replicas do.
- More unique guns on their side. The rifle, grenade launcher and lightning gun are cool and all, but the rifle is literally indistinguishable to the vanilla one, the grenade launcher is buggy as fuck and the lightning gun is okayish. Why not have stuff like revolvers, machine pistols and a couple experimental weapons, like a triple barreled shotgun or a freezing weapon of some sort?
- More unit variety. The Replica had a perfect balance of regular dudes, power armor and mechanized units; The Nightcrawlers are all just guys in red. Only one out of three actually do anything different, and that difference is that the enemy has I-Frames sometimes. Make some unique robot/cyborg type enemies, since it seems the Nightcrawlers have more technological prowess than the Replicas.
- Have them as a rare but deadly opponent. In PM, they basically replaced the Replicas later on in the game. That's nice and all, but they have little variety among them (unedited) and their personality doesn't carry the fights like the Replicas do. Make them appear as like miniboss squadrons if that makes sense, breaking up the fights between Replicas and Alma's spirits.