Ok, I’ll try not to go off-lore this time.
My suggestion comes after the Stephen King book: “The Mist”.
We are talking about a military base where experiments with inter-dimensional portals takes place, but something goes wrong, and this “xeno” dimension break free and starts expanding in the nearby locations, bringing along monstrosities of every kind.
So the lore talks about a prototype of teleportation device that opens up a portal to an external world.
(maybe I’m wrong, but I know that there are portals anyway).
So maybe the lore can be “Expanded” with an additional extra-terrestrial dimension that got released onto the earth from one of this portals.
This mist will be evaluated as weather that occupies a little part of the map (4x4 sectors for example), this mist actually moves around at a slow peace, and inside it there will be Monsters that can’t be found anywhere else.
The mist, as the rain, will not enter any kind of building.
Technically this monsters can’t leave the mist, because it may contain a particular chemical substance that is essential for them to survive, if they leave their natural habitat(the mist) they will become weaker and less effective and if they stay out for too long they will eventually die.(Maybe this can be worked on a little)
For those of you who have seen the movie or read the book, I’m not suggesting the implementation of such OP monster, as the hentai tentacle monster that lurks around the store, found in such a masterpiece (bad luck that spiders and giant was was already implemented in Cata) but maybe we can find other kind of creatures that resemble more to an alien world, maybe opposed to the nether ones, creating a status of permanent war between these two universes.
I am full of ideas and propositions, and everyday I will try to fit them into the lore to make them acceptable, if the fact that I post so many content suggestion bothers you, let me know and I will regulate myself.