Not just encountering the same old NPCs we've been shooting at more often, but a larger variety of brand new enemies. Like Super Mutant raiding parties wandering way beyond Black Mountain, gangs of wild robots, brand new mutated Mojave wildlife that wants to eat us (and likewise we could eat them), power-armor clad mercenaries hired to actively hunt the PC, cannibal tribes, crazed cultists, and even more different raider gangs?
Not only that but augment the current lineup of existing NPC hostiles with more variety, like Fiends who have taken so much Buffout they are nearly as rediculously huge as a Super Mutant, or Super Mutants decked out from head to toe in a completely enclosed suit of metal armor, Caeser's Legion Frumentari equipped with Stealth Boys (like the Nightkin), Two-headed and Four-armed Deathclaws, Cazadors that can shoot their stingers, etc. etc.
Those are just some ideas. I actually don't care over the specifics, I'd just like a DLC that makes the Mojave even more combat-oriented for more combat-oriented characters so we can play with all the awesome toys we've been given in GRA.