Well come up with more ideas yourself first to make it more than just a zombie shooter and it might be of interest to discuss ideas with.
Right now the only thing I picture in my head is a bunch of feral ghouls going "HRRR????AA!!!!" and then cue the bullet storm.
I have no idea of what to suggest to save this pitch from being a shoot 'em up only.
So let's do this, I ask some questions and you think up some answers to flesh it out:
Why are there tons of feral ghouls there anyway?
How come some ghouls have mutated differently?
Why is there someone in this city?
Who is this special someone?
Why is he important enough to risk going through an army of feral ghouls?
Why are The Courier contracted?
Who's the other merc?
Why only two mercs on a suicide mission?
If this person is really important then whoever wants to get a hold of him could easily get enough money to buy a dozen mercs, so why only two?
If it's because that the contractor doesn't have a lot of money then why would the other merc want to risk his life for this mission?
A nuclear facility of some sort that put out massive amounts of radiation during the Great War, maybe something like Yucca Mountain in Nevada but obviously not the same facility considering that it's 100 miles away from Las Vegas. A fictional facility which turned many people into Ghouls and Feral Ghouls which soon consumed survivors of any relatively close cities(Far enough not to affect NV), what city this would take place in I don't know considering in the Fo universe real places are used and I'm not a native inhabitant of Nevada so who knows. There are a group of Ghouls who have hidden out under the city for years whom you could choose to help and in turn they could help you find the person you are looking for.
The majority of the story and dialogue would take place with the Ghouls taking refuge, and also with the mercenaries and survivors of the treasure hunting group.
The person could be a rich treasure hunter who has reason to believe there is something valuable in the city and traversed into the area with a team to find the area more dangerous than they thought, as the only survivor he/she finds a place topside to radio for help. Maybe they don't do it soley for the money but the thrill and hires mercs to help his missions. Hell maybe when you find him he's some weird Feral Ghoul hybrid who is just luring people into the city for prey!
Some Ghouls have mutated differently because not all we're exposed to the same amount of radiation, some in the city just turned into Ghouls while those closer the plant went feral.
You would run into a dozen or so wreckless merc's who are also looking for a good payout, there could be double crossing involved and maybe some companions. You would have to choose who to trust. The mercs could be really tough as only someone crazy would respond to the signal. And obviously there would be other enemies to fight other than Ghouls like radiated animals or Super Mutants somehow, not to mention other Mercanaries. Also you could run into survivors of the treasure hunter group. Maybe new Feral Ghoul enemies also? Bring back Albino Radscorpions?
Also the area would be extremely dangerous so you would have to strategically map out your plans and routes to take.
How you could get to the city I have no idea, would it be possible for Ghouls to travel far enough to whatever city would be close enough to a nuclear facility? I have no idea. But considering Zion National Park is over 150 miles away from Las Vegas and you and a caravan travel to it in 5 seconds it's not really that big of a stretch.
I'm not a professional writer nor ever proclaimed to be, I'm just throwing ideas out there for fun and thought maybe others had their own ideas for a DLC they'd like to see. Apparantly I have to have an entire backstory that ties in with the Fo universe otherwise people will just hate the idea. But it was fun thinking it up so win-win.