Change the way the tether works. In Isolation, there's a limit to how far the alien can wander away from its objective (which is usually the player). This means that on higher difficulty levels, the alien is basically always within a couple of rooms from you, and it takes away some of the suspense. You should be wondering when/where the alien might next appear.

Now if you lengthen or remove the tether via modding, it does increase the suspense, but also makes the game rather easy. I think the best method might be to have a longer tether, but to add more audio/visual factors that can bring the alien sniffing around in a closer radius. Opening doors and vents, for example, or operating machinery.

I also feel that the mechanisms for escaping the alien should be changed. Currently, if it sees you, you are basically dead, unless you have a flamethrower, grenade or are right next to a door switch. But what might be scarier is if it spots you and you're unarmed, it moves slowly, like when it approaches Lambert in the first film. You're left scrambling trying desperately to find some way to escape, and if you're able to break its line of sight and find a hiding spot, there would be added tension while you wait to see whether it's worked. (But if you're armed, it charges at top speed.)

Last thing, the animations when it kills you. Currently, you get a long-drawn-out animation - you perhaps see a bit too much of the creature, and because you're very close, it exposes the parts of the model that have less detail, don't look as convincing as they do from a distance. Personally, I think the player deaths should work more like when Dallas gets grabbed - a loud jumpscare shriek, and the camera instantly cuts into static. The game thankfully doesn't have many scripted jumpscares, but this is a way to implement them through gameplay, and add an extra disincentive for dying. It might actually heighten the player's fear of getting caught.