I'd like to see an armored lab coat introduced in a DLC. Not, like, the Armored Lab Coat like a lab coat that's been given ballistic weave or something, but a lab coat model with a combat harness and maybe a shoulder pad or something on it. I like the image of a scientist out in the wasteland, but the lab coats that are in-game right now look to neat to be good adventurer-wear. I miss the lab coat over a Vault jumpsuit that was in Fallout 3. Honestly, I'd like to see more Vault jumpsuits in general.
[Fallout 4 Institute/main questline spoilers]
What I would like to see, besides new lands and more story content, is Fallout 3's Dr. Zimmer. In the Institute, they talk how he is away in some sort of secret mission for the Institute and Dr. Ayo is stepping in for him. I think it is a very good opportunity to make something out of that situation, with Zimmer's reaction to the player depending on how your relationship with the Institute transpired.
I'd like a new locale, similar in scope to Solthseim in Dragonborn. A good narrative that is open to interpretation (they forced us into the concerned parent role for the main game, but they have another chance in the dlc). New clothing, hopefully more that works under armors. Speaking of which, more armor in general. I'd like to wear armored hockey pads, or something that looks strewn together. The ability to wear a bandana in my hair would be nice, as well. Make the questline have diplomatic routes (which were lacking in the MQ) and throw in some more HP Lovecraft stuff. Adding new stuff to screw around with in the settlement crafting menu would be nice, too. I'd also like new weapons (really holding out hope for a bow/crossbow, but I'm not going to get my hopes up) and some new facepaints/tattoos. I think it'd be cool for them to set it on Martha's Vineyard, or Nantucket.
Yeah, heading to an old vacation spot would be a great chance to have fun with the changes the Great War brought. I agree with you that DLC brings some great opportunities to open up the story a lot; you really are forced into a specific role in the Main Quest of the base game, given the origin story for the player character, and without needing to conform to that path, the DLC can be about...anything, really.
More fun armor and weapons is, of course, very welcome.