So suspend our sense of disbelief for a moment, and imagine that despite its impossibility of ever happening, Bethesda has allowed Obsidian to remaster New Vegas, and additionally produce one more DLC.  What should the DLC cover?

My vote would be something involving the NCR capital of Shady Sands, and a legion city.
My reasons for that is that the player basically has to choose between four factions.  Independent and House are shown to us, but the lives in Legion and NCR territory are not shown.  
I'd love to see a legion town where there were women who weren't just ragged, bent-over slaves moaning under the weight of heavy bags and the men were more than violent kill-bots repeating the word "profligate!" over and over.  If Cass, a very independent, heavily armed, heavy-drinking woman could roam legion territory doing business, without being immediately shot and/or enslaved, then Caesar's territory is a little different from how it can appear when only listening to their insane kill-bot legionaries.  They did have a society, an economy, etc, but we never see it.

People who didn't play the older games don't know how advanced the NCR is (or was, at least.)  All we see, again, are the under-funded military, whining about nuclear winter.  We don't see a huge city with paved roads and new construction!!, we don't see the doors so advanced that they're actual force-fields, a huge population that would put the rest of the world to shame.  Their police are dressed in new, non-ragged blue uniforms heavily resembling modern real cops (as far as could be told from the pixels).  Shady Sanders don't need to be squatters in two hundred year old crumbling ruins covered in trash like the rest of the world seems to live.  Even goodsprings has trash on the floors in many of the houses if I recall.  They have brooms in shady sands.

Funny enough, there actually was a large portion of Caesar's territory cut from the game due to hardware limitations and the short dev time that Obsidian was under.

Modders have been so industrious with dlc-ish big mods like beyond boulder dome. Maybe someday we'll see something like that on a grand scale.

The only thing we'd have to worry about would be that many of us probably have ... differing... ideas as to what legion territory or Shady Sands might look like, so there would be a lot of pressure on the modder maybe.

Something Legion related hands down. Maybe an infiltration type deal, undercover deep in Legion territory.