I know a post like this was already made but I thought it might be a good idea to come up with more ideas since Bethesda did announce that they want to make to dlc based off player feed back and what they think we want. anyway here are my ideas
My first Idea going out west back to the NCR my main reason for wanting this is because the game references back to the NCR in a few bits and well to those guys who say it's not possible because the Legion, well even if the Legion won the second battle of hoover damn, in the end the NCR would win the war with heavy casualties but the in end they would win regardless of the circumstances of the Currier 6's decisions in New Vegas. Reasons for this being well one Legion soldiers are trained to discard advanced weaponry the moment the battle ends, NCR soldiers are trained to think for themselves and they have Vertibirds now (basically air control with heavy weapons attached) sure the legion has an anti-air artilery canon but they only have one of those. Also Vertibird travels turns week long journeys into a few hours. so it is reasonable to assume that travel by Vertibird could turn the multiple month long journey it would take to get to the western wasteland into a few weeks and the brotherhood has Vertibird so I can imagine the excuse being Maxson or someone in the brotherhood trying to contact the west coast brotherhood. Also for those who don't know Fallout 4 takes place 10 years after fallout 3 and that means 5 to 6 years after new vegas so I'm pretty sure the NCR's war with the Legion would be over by the time Fallout 4 takes place even if the Legion won the second battle of Hoover dam. War's usually only last about 6 to 10 years in RL and then a new one begins. Also I want to see Bethesda do this because it's unlikely Obsidian will make another West coast fallout game with Chris Avellone leaving the company.
Another Idea I had is to have a form of the Enclave return as like an optional join-able faction in another area because honestly in the Lore it is stated that the Enclave has bunkers and vaults made for Enclave Personal all across America and we're going to believe we finished them off at raven rock. I personally don't believe that because statistically not all of the Enclave could have made it to the capital wasteland in time and as I mentioned they have bunkers made for them all across the American wasteland for all we know there could be a bunker with cryogenically frozen members of the Enclave before the great war and like maybe the computer messed up because of a bomb going off near it and never revived the Enclave members who are in cryo-sleep, and maybe you accidentally or intentionally revive them not knowing who the Enclave Are. Also Your character is a registered American Citizen and a veteran soldier from before the war so that could be reason enough for them letting you join. I could see this happening as like a main Story Expansion back or like something that could happen in like a trip to the New York wasteland.
Lastly I think that with each expansion pack they should add stuff to the settlement creation like maybe as a way to expand your settlement but not increase the lag they could make buildings and areas that allow you to go inside via loading screen and they allow you to make more stuff in there like the house in Diamond city or maybe allow in to start building underground, it would probably use alot of Junk Resources to do this but I think it would be an inventive Idea to get around the lag the comes with removing the building caps/limits in settlements