Something I found while messing around on stream was a softlock you could do in the Ruins during the Predator campaign. When you've acquired the mask and are leaving the Predator only section and get past the caves, a squad of marines run out from a door ("Door's closing. Watch the roofline for xenos!") that closes shut behind them. If you move just quick enough it's possible to get through the door just before/ as it's closing and get stuck on the other side.
Here's the softlock and some of the areas behind it: https://streamable.com/dldf8
Unfortunately not only is it kinda finicky to pull off but nothing past those doors actually spawns except level geometry. I've tested out everything I can think of, and I can't manage to get the game to progress after that part. You can go back up to the door you're expected to come through but since it's locked you can't get back and since the lock is on the other side you can't interact with it.
Although you can get into the arena nothing happens when you do. It has very buggy jumping for some reason: when you're inside the arena you can focus jump to platforms at your same elevation or below but not higher. Because of this I haven't tested what happens if you try getting into the observation area you jump to when finishing the level since best I can tell you can't. I'm pretty sure it's just a straight softlock and the only way out is to restart from the nearest checkpoint, which just puts you back in the caves.
Anyway, I noticed at the start of the Ruins level for the Alien that you can see the two archaeologists entering the building and the door locking behind them. It's a serious pain in the ass to pull off right now, but I've gotten it (semi) consistently after enough practice that I feel it's practical enough to attempt in a run. Essentially with optimal movement you can get inside the door before it closes, and unlike the Predator you don't softlock because the generator you need to bite to progress spawns.
Here's an example of the skip: https://streamable.com/uia9v
Obviously this bypasses needing to jump down the vent, hit the first switch, wait for the gates, hit the other two switches, crawl through the sewer, then finally enter the building to save a ton of time for such a short run. In theory it saves so much time you could try and fail it a couple times before you'd be better off just going for the switches. Thankfully it's right at the start of the level so it's super quick to practice over and over until you grind it down to something consistent.
So far the most luck I've had with it has been:
1.) Sprint to the ledge immediately after spawning.
2.) Tip over the ledge and jump down to the ledge just overlooking the water pool.
3.) Look over FAST and jump as far as you can towards the wall of the staircase over the pool to your left.
4.) Sprint should have recharged now, sprint directly towards the door.
Once you're near enough to them while going fast you get the prompt to harvest them, but I haven't actually tried doing that yet since the time it'd take seemed pretty counter productive to me. If it helps pull you through the door more consistently it may be worth doing. I also had one instance where the door closed exactly on top of me and I was stuck in place for a while. Spamming jump and sprint while looking around towards the inside eventually got me free and through the door, so there's still a bit of leeway in how precise you need to be, definitely not TAS levels of difficulty or anything.
Here's me getting stuck but still making it through: https://streamable.com/jz8th