It's unfortunate this died due to IP issues. I'm going to make up some art for monsters that are similar to Aliens and Predators but different enough to be its own IP. I'll make it open source for hobby game devs to use if they are making a free game. "Xeno-beasts" and "Galactic-skull-hunters," or something similar. I'm honestly shocked there aren't any open-source alien species that can scratch the same itch as the xenomorph or predator that are different but still similar enough for RTS and style games that don't run the risk of being shut down by corporations.
They have to look different enough so that they aren't too easily mistaken for the alien or predator IP, and need some differences in lifecycle. An example of a difference that can work would be having concentrated sodium hydroxide for blood. This is not an acid but rather a base. From a functional standpoint, you still wouldn't want to be sprayed with it as it would leave severe chemical burns. In a video game, the player would want to avoid it and stepping in puddles of it.
Galactic-skull-hunters likewise need to recognizably not be a "yautja" but still let players have the space monster going hunting for humans and other space monsters feel.
So, how close can we get with copyright laws? Well, look at some precedents. DC Comics was able to make Doomsday despite obvious similarities to the Incredible Hulk. Look at Superman compared to Omni-man and Hyperion. The archetypes can be obvious, as someone cannot copyright an idea or an archetype, meaning 20th-century studios do not own the concept of a parasitical space monster. Copyrights generally exist for stories and the characters that exist in them. So, from that standpoint, a monster is legally a character that is part of a story owned by an IP holder. But, if no elements of the story are used, meaning no mention of LV-426 or anything like that, and again, if the monsters look different enough as well, then it does not infringe on the IP.