I've got a few ideas here, let me know what you think!
1. Moon Trip - teleport to the moon and find the settlements left from the battle before the great war AND find the Brights from NV there too.
2. Nick Valentine Prewar Memory Simulator - perhaps some case that you can start in prewar Boston and finish in 2287.
3. Underwater Mission - not a lot thought for this one. Heard about the harpoon and the underwater structure.
4. Enclave in Rhode Island - was looking forward to some enclave presence so this is my hope for a DLC.
5. D.C. trip- who wouldn't want to visit the capital wasteland?

1. Maybe, as long as it is done better than Mothership Zeta.

2. I fully expect DLC to tap into the full potential of The Memory Den (I was very disappointed with it's meagre use in the game so far)

3. Same as number 2, I expect there to be further content to expand on all that expanse of water.

4. Hell no, leave the Enclave alone already! I could accept a small remnants force similar to F:NV though.

5. No again, DC would be boring to me now. Plus I don't think Bethesda would want to make any one ending or set of actions canon.

We've already had space. Underwater would be a better equivalent to space. Similar in the way of living quarters and vehicles, but will be a newly uncovered idea.

A space station or moon base would be cool. Commies in SPACE!

Some pre-war Boston things, in a film noir type of Nick Valentine style would be awesome.

Underwater Vault Tec base would be cool.... because seriously, wouldn't these guys, who made vaults, of survived?

Enclave.......... no..... done...

D.C. trip.... maybe, as long as they would redesign the entire thing.

Being able to make and style your armor and power armor would be nice. Also one place I wouldn't mind seeing in the common wealth is snow accompanied by green houses. Also a means to expand the boarders of sanctuary and other places to an extent. Perhaps a caps based mechanic. I would really like to actually fly a vertibird... I was very sad when I couldn't. Also a way to even out the landscape for farming would be VERY nice as well . Oh and naming settlers and dogmeat.

I think, although we have had a space DLC, that a DLC travelling to the moon would be awesome. I for one think it should be much like The Pitt or Point Lookout DLC that open up a whole new map to explore with main and side quests, complete with it's own weapons and armours. For instance there should be an astronought style power armour, as good if not better than the X-01 Mk VII...Because let's face it, It is a new area why not have better equipment? And being that it's on the moon, give us some kind of transport like a moon bike or Rover.

Nick Valentine Pre-Great War is a little impossible being that he is a Synth made AFTER the war...perhaps delv into the playable characters time in the military? Maybe a significant Tour of Duty? Like the old Anchorage DLC? Maybe have it tie into the Enclave? Where, when you come out of the Memory Den, you seek out the remnents of the Great War? Whether they be generations of descendants or Goulified Brothers in Arms, giving you the option to either help them rebuild an army or crush them like in number 3?

The Vault Tech Underwater idea would be awesome, I can sense a bit of an OLD WORLD BLUES feel about though...although...again a new Oceanic Power Armour does sound appitising...

And as for a trip to D.C., perhaps a Broken Steel atmosphere where the DLC is only accessable after the main story line so that whichever of the four main factions you side with send you to D.C. to help set up (or maintain for BOS and Railroad) a branch in the Capitol...

I dunno, just my idea...I know I referred to a lot of the old DLC but they we're so good, why not use them as baselines?

For the enclave? Just like "the remains" we see in Fallout New Vegas

Nantucket

... and one more.

Somewhere else.

I actually think an Enclave DLC shows some promise. However I don't want them to have a physical presence, instead I'd like them to take the same role as Vault Tec ruins, that is exploring abandoned bases and underground labs. Uncovering experiments and Secrets long held therein, and finding some awesome tech along the way. If you think about it, it would be logical that the Enclave would have at least a few bases in every major state that housed important members of society, Congressman or Former Presidents anyone who had or held an office of power so they could have been extracted later on, plus various labs for specialized tests and whatnot.

Exploring eerie ruins long forgotten with unknown threats posed by security or some abominations left behind could be good fun.

Actually I think a good DLC would be a cyborg option for your character. I would like to see us able to add robotic parts to ourselves to a way of upgrading. I had the idea of making it that when you are at the overseers computer you can find a file on it talking about another vault testing out these parts on people in the vault who have missing limbs. To see if they can survive the radiation with these added implants. This would cause you to get a map update and a new mission, it would lead you to the vault and allow you to fight your way inside or see the vault untouched and filled with cybernetic people and find a scientist/doctor who was working on these parts. You walk in and go around and speak with the people and find the scientist allowing you to get some cybernetic implants as well. I was hoping we could do this DLC since so far I have not seen any fallout games allowing us to be part cyborg or be a synth in the game which I think would be cool.

I think there should be a dlc based expanding the role of the Minutemen quest line that would involve the opportunity to Unite the commonwealth to a new government under the Minutemen turning them into an army like the NCR and you could be president

Or

An expansion to the brotherhood that allow you to be elder