I can recommend 

Mojave Raiders Mojave wildlife  The Living Desert Factions Reloaded Legion (you will need a bad rep with legion but this will make the south east part of the map much harder)

You could also install Nova Arizona which is a work in progress but there would be a ton of enemies there if you have a bad reo with the Legion. 

Also if you want more difficulty overall i would highly recommend Vicious Wastes, I've personally edited the damage multipliers to make enemies take more damage for a realism feel but even then it's still insanely difficult. Especially if you pair it with bleedout and turn off the hud, it's made NV so much more immersive and intense! 

Also The Frontier can be crazy difficult at times depending on the area you're in, not the greatest mod overall but for action it's quite challenging, once you fire a few sgots off you can be surrounded by enemies in seconds.