I don't really see NeN getting DLC at this point, there is not much else you could do with it since you had a very set concept and goals and you met them already. Extending the story would feel like stretching it.
Only thing I'd say would be possible would be a CHEAP dlc (like $1) that unlocks the cinematics for the alternate endings for those that have beat the game at least once. Hell I'd buy that. I am sure there are a number of folk that finished the game but didn't have it in them to go and play through the last sections to find the divergent paths for the alternate endings and play through those. Then you patch the base game to link to it in steam or whatever service when you see the "path" screen showing there are alternate endings. Will folk gripe? yeah. though at this point it would be obvious it wasn't your goal initially.
I grew up before DLC was DLC. Heck before there was a thing called DLC at all. Tyhen we had expansion packs, large usually "bang for your buck" addons for a game that cost the price of a budget title and gave you a pile of added stuff to do in a game for a while. Back then, those were actually generally received well and people liked getting more to do with games, it felt like the developer cared about the game they made and the fans that bought it.
Nowadays the exploitation of DLC has soured the concept quite a bit, but it is here to stay and part of most game development models. I was actually surprised when the witcher 3's studio announced the 11 dlc they release will NOT be paid for, but all entirely free. That kind of decision gets them a lot of fan love, but love can't pay bills.
For the next game, it COULD be an addition if it is either fairly trivial and there just to enhance some players experience in ways they like, or if it's significant and seems worth the cost. So there is a lot to consider and the way NeN is, if the next game is like it I don't see a large market for DLC.
If you were to do DLC the kinds of things I'd be interested in (obviously I don't know the next game's design so I"m speaking speculatively) would be:
Alternate main characters (Like if you had the option to play NeN through as Gabby and see it all from her eyes somehow, though i realize it wouldn't make sense at this point)
Alternate Soundtracks
Prologue level (even though you play it after the base game, it gives additional insight and hints at the story and characters to better give the experience weight)
Epilogue Level (how does the experience apply to the main character? did it have a good effect? a bad one? None? )
The Real World level (If the nigthmare is a twisted reflection of their life or history, offer a level that clears the looking glass a bit and takes away from of the distortion. This might be in lieu or a pro/epilogue and condense the real world reflections into 1 chapter)
Optional filters for game view (e.g. sepai tone, old film style, harsher shadows, level/nightmare specific gels, NeN 1 shading, background texture so the game looks like it takes place of a large piece of art paper)
Optional 8-bit NES mode (lol)
Big head mode
Hardcore Gameplay Mode (where you add back in elements you think players would find too difficult, but those that want it can enjoy the challenge)
I don't mind alternating the texture of the experience in different ways, and I love alternate or additive experiences that make the game feel more significant. Granted they might not fit the initial vision "perfectly" but it can give the game new life, and everyone has their own preferences and might appreciate the game altered a little.