Maybe aliens were on earth before cataclysm? Imagine, they came to earth to study its lifeforms and human society, but cataclysm happened and they were caught in it like humans did. And now they are scratching their heads(?) and thinking wtf has happened here?
Resonance Cascade! Horrific Cyberdemons invade!!
Survivors rejoice, more cybernetic enhancement modules to carve out of fleshprey.
Really, some days I wonder who the real monster is.
I’d be tempted to think that any nether or otherworldly invasion is simply a preventative measure-- I mean, look at the world as it stands-- the dead have risen, chaos reigns… It’s a world where only the strongest survive-- and the survivors are almost more monstrous than the monsters.
Think of it like this: “you see a gracken. Its hands are a blur!”
But… what does the gracken see? “You see a survivor. He has horns, hydraulic muscles, whip-like tentacles in place of arms, and a chitinous exoskeleton. He is wearing a gas mask and scratched heavy power armour. He is wielding an M4A1 with an underbarrel grenade launcher.”
Frankly the gracken (even though it’s a bit of a parody ref) scared me a bit the first time I met one. something with hands moving too fast to see? sounds like it could rip you apart while you’re not looking. Of course, Mr. N. Batman had a few choice words for him.
Anyway, well played Nietzsche. Well played.
Does fighting for your survival and prosperity innately make you a monster?
I say nay.
We are not monsters, but just the same, they are not either.
We both exist.
We will do what we must, because we can.
For good of all us, except the ones who are dead.
Maybe some kind of District 9 alien? Where the ship with the aliens started being destroyed and the ship itself began to travel for the most near habitable planet, in this case Earth.
Quoting “Big Bang Theory” here - “In an infinite number of universes, there must be an infinite number of Sheldons, therefore…” 
So, nice throw Mr. Creator, granting us half-a-dozen dimensions we cannot seem to participate in or influence in any way. But other Sheldons can. Lemme just point out the brilliance of spontanious, the wicker of the common sickness that the Fallout series introduced to the unexpecting; the two-wordly word, post-apocalypse. Everything that’s too gruesome to be encountered is either ommited, or extremely rare. Or, the way a vivid, imaginative mind sees it, simply has no similarities left to concern itself with ruins, scraps, putrid ghouls and pack-animals the Mutants are. The nut in that shell is better_adapted_entity. It senses portals, it prays on the most dangerous creatures known to men, draws upon and releases weird and complex energy streams. It can guess the presence of an another world, that paralells in a way with the decay that surrounds it, but it can’t tell who’s watching. And the next in the evolution chain IS - the supermutated human. He will seek, he will learn and follow, and stumble upon the Cataclysm Earth. And like on his own world, he’d seek only those means that support the existing hostilities with those not of his origin, and look only to conquer.
Truly, an alien from Hell, and beyond. 
I like the idea of aliens noticing the dimensional rift as a major threat and acting accordingly.
Maybe the netherworld is merely a medium and they’re on the other “side”, somewhere else physically inside our reality.
The thing is, we already have the nether mobs and the blob itself as a bunch of truly inhuman enemies. And if the aliens are indeed too “alien” then they run the risk of failing to elicit an emotional response. We don’t need them to be another bunch of zombies, or people, or nether beasts; we also don’t want them to be just another bunch of goons rushing at you in all directions.
Personally, I think aliens arriving in order to complete an understandable objective (say, quarantine the planet) would be a welcome relief from the grimmy grimdark of grimdark that so many of the other mobs convey.
Personally I think that a race similar in appearance to the Prawns from District 9 would work well. Alien, hard to comprehend, but not so much as to throw off any connection the player might make with them.
maybe the slavers theory can be used
they travel the universe in search of “primitive” life forms and slave them all to work or turn everyone into zombies they can control somehow
We already have Polyps, why not add Yiths that are embittered about losing a potential home?
I think a little bit of hitchhikers guide would be nice. After all, galactic civilization should be perfectly fine; the main thing about aliens should be that they’re worrying about space yoga and overtime while we’re dying over pencils. Forcing them to be as grimdark as everything else just makes them less unique.
I agree with observing from a safeish distance. The aliens should fear only the tippity-toppity of the monsters, but they should instead be plagued with much more mundane concerns; surprise inspections, strikes, petitions to save a monster that looks cute to the inhabitants of gora gora 3. Every now and then they should be faced with actual problems like a lower tech level civilization that wants a piece of that dimensional rift, crashed escape pods or the odd fugitive trying to lie low, but on the whole they should require you to use a entirely different, pre-cataclysm part of your brain when your dealing with them. If it’s the kind of problem that would arise in Rick and Morty, it’s the right kind.
Dang, imagine if alien ships were vehicles made of entirely different materials than other stuff… and you could salvage stuff out of them :3
But the materials are to hard for you to machine. So you have to tie them down, or use preexisting holes.
High enough explosive/ chem skill lets you make bolt holes with hit/miss success.
hmm… Im wandering into bad ideas thread territory.