If I were a real modder, I would make a total conversion to something inspired by an old turn based game I used to play, Spellcross. Basically, inter dimensional portals started to open worldwide and humanity gets under attack by a massive amount of creatures only know in fantasy tales like dragons, orcs, demons, undead, etc. Surviving human armies band together as The Alliance to fight the new menace know only as The Other Side.
So the game would be set pretty much in the middle of this new cataclysm. With civilians moving towards military bases, barricading themselves at home or just surviving, military forces engaging these creatures, securing towns and saving people. Orcs, goblins and other sentient creatures wielding medieval-like weapons and armor attacking and abducting humans to sacrifice them to their gods and wild animals and monsters attacking both sides.
New games would probably start in a similar way to Cataclysm. As a survivor in a random home. Playing in the same world after a while will unlock new options, like starting as part of The Alliance or being a survivor from another race. It should depend on story progression.
As the time passes and you manage to survive, or start new games as other survivors but in the same world generated, new things start to happen:
-TOS forces uses a heavy screen of fog/mist thru the use of magic to hide hordes moving to attack the Alliance or settlements. The Alliance will notice this pattern and try to send their own forces toward this weather phenomenon to intercept.
-TOS forces will start to bring more deadly enemies like demons and magic wielding enemies and the weaker enemies first encountered like Goblins and Orcs will learn how to use human guns and items.
-The Alliance will start using melee oriented soldiers since bullet proof vest and monsters engaging mostly at close quarters will force them to adapt.
-The arrival of magic in this world also sparks the ability to use it in humans after exposure to certain elements and training.
-Some humans start to go rogue and form raiding groups to attack anyone for survival or start to worship TOS and defect to join their forces.
-Some Portals fall under Alliance control, allowing them or people with the proper papers to go thru them.
-Goblins and Orcs are labeled as weaklings by Demons and other superior races and are no longer part of TOS forces, forcing the ones already in our world to go rogue, surrender and form communities, join human communities or joining the Alliance as soldiers.
And stuff like that, yeah. 
