It would be cool if there was a creature that stalked the player throughout their play through, maybe looking like a xenomorph or something unnatural/alien. Keeping to the shadow, being stealthy like not hearing anything or seeing when it goes through pipes (like no more the colors that appear on the pipes). And slowly getting more brave after a cycle, eventually hunting the player and chasing after them periodically. It could be a really cool mod that will add a twist to your play through.