So depending on my ending and what it entails whether this or that is still alive, etc..., will this matter for playing DLC or will DLC be completely separate and not matter who what you chose for the ending?

I don't think that will matter much in Far Harbor because it is so far away (could be wrong though). I do want the last DLC to be like Broken Steel where we get to see what happens next with our chosen faction though.

I'm hoping this as well. I've got the "Best Ending" saved with all companion perks, mags, side quests, (and now nothing to do) etc.. as well as my personal favorite ending with Father so as to wear Maxson's coat with Final Judgement. I could totally play the rest of the game wearing this and using this gun with absolutely no care on any other outfit, weapon. I love it! Now I just have to figure out which ending to play from...(and I know many would say both but which play first for the best experience?).

The Far Harbor info makes it sound like the Pitt, where you decide the fate of that region but it has little effect on the main game.

I doubt the main game will have much of an effect, if any at all, on Far Harbor. At the very LEAST, I can only imagine a minor reference to events in the Commonwealth.

Actually, I wonder how Far Harbor would go if Valentine dislikes you and is hostile to you.

lol. That would be very interesting to see indeed. I don't know what would be funnier, suddenly seeing Valentine all nice and amnesia like or if he comes with heavy regret to you and offers you the job. Interesting indeed.

I'm not even sure how that would work given that there is no guarantee that the player will have done anything significant in the Commonwealth prior to Far Harbor.

As to Nick, he may not even be needed. The case could simply be presented as a folder on the desk in his office and you might be able to do it before you even rescue Nick from Vault 114.

considering you never met Nick before FO4, they can't do a prequel. And if Nick is hostile towards SS due to events by player, or by choosing wrong faction ending, I kinda think you may have to play as pre-determined character that maybe works for\with Valentine. Or as a person seeking him to help work the case, but also a new character.

Will the new landmass be accessible from Commonwealth after the DLC is finished?

Ideally, I prefer the DLC's happen separate from the main story and events. Personal preference I guess.

Events that take place away from the other action, so playing them before or after the ending isn't a requirement.

I personally don't think Far Harbor will tie into the main story so much, but it will obviously work in some of the main story themes. My biggest hope (and Bethesda's biggest challenge) is making things feel distinct but integrated. For instance, I think open world games are going to continue to push towards having more fully realized worlds, and that entails a great number of things, but having NPC awareness of events is crucial. Otherwise, it feels like we are always operating in a vacuum. To that end, I do hope that if we start the DLC halfway through the main game, that our faction allegiances will be noted by NPCs in Far Harbor and will impact how NPCs react to us (assuming they know what factions we are in). Similarly, if we have already completed the main quest, I hope that progress will be marked by NPCs (much like the First Dragonborn makes comments on whether or not you've killed Alduin in Skyrim).

Beyond just being aware of what we've done, I hope that what we've done will also impact the NPCs; netting us friends or enemies depending upon how they feel about our actions. Overall, this is all highly dependent upon what the DLC will actually involve, story wise, but I think you get the general idea of what I'm driving at.

I too hope for a dlc similar to Broken Steel, where we see the continuation of the greater story of the Commonwealth. It would be incredibly exciting and add to the replay value extensively, to be able to participate in further shaping the future of the Commonwealth, and I hope they might use the DLC to polish up end game events and make the world feel more alive again.