So, giant horror fleas from nowhere?
There'd probably need to be a flavor reason why they were around, but yes. Something like that. The only reason I added the bit about summoning them into the world yourself is because it would be a little cruel to release an expansion with only optional endgame content in game where people can ironman themselves into only being able to play the last mission, yet lovecraftian aliens wouldn't quite fit with the feel of the current aliens. Thus, add a way for the player to call them into the fight whenever. Which should probably cost money to balance out the potential exp gains.
Designing interesting new Xcom aliens is hard. I've given it some thought because of the possibility of me sometime modding X@COM. They've covered a lot of their stereotypical alien bases, but at the same time have a very unique style. There's a reason the remake only has 4 new aliens, and 2 of them are variants on Mutons.
-snip-
This is my source. Specifically the trivia at the bottom. The only source cited has Soloman quoting MIB as an influence, with no mention of slenderman or lizard people.
I could see it. Let Vahlen have her way with an Ethereal, and you learn about "the coming threat" early, as well as enough information to build a beacon that would call them. You can build the beacon, but the Council deems it too dangerous to use, regardless of their opposition to the current aliens.
Then, if the Doom Counter fills up....
You posted this just before i posted, so might as well respond here. That's a really good idea! I've always thought its annoying how you can win your fights but still lose the strategic layer because you didn't spam satellites hard enough. This way you would still be punished for triggering the doom counter, yet have a chance to save yourself. The main problem is that because the hardest part of the game is the early game (in my opinion, the difficulty reaches its peak at the first council mission, or the first terror mission), most players would have to intentionally lose to fight the horrors. Perhaps any live psychic alien could be enough? Or maybe you get a special mission if the doom counter advances to 4 that lets you get what you need for the beacon research.
For my totally made up expansion I mean. Maybe in the far future I'll complete a programing project ever and it'll be this as an X@COM mod. That's the in-development XCOM RL on these very forums if you guys somehow don't know about it.