I think Fallout 4 should be in the Massachusetts Wasteland. This Wasteland will cover the Commonwealth states, along with New York being on the Western Border, the Institute being in the middle, New Hampshire/Vermont being in the North border (I don't know what major cities are south of those states, so I can't give an example), and Rhode Island on the South East corner and Connecticut on the South Border.
Now, lots of people are saying New York City was completely destroyed... what they should do is only put in the outskirts of the city, since the Nuclear bomb didn't reach the whole city, so maybe a few tall buildings survived, and if you go to the New York City outskirt buildings, climb to the top of them, you can overlook the crater that once was NYC, which would be a cool place to see.
The Institute, from what I read on the Vault, was completely untouched. They are one of the most, if not, the most advanced faction, with their cutting edge technology and development of Androids and Synths. It would be cool to see what kinds of armors/weapons they have in store for us to loot and what-not.
From looking at a few pictures of New Hampshire and Vermont, I could see them being more on the Country/Swampy sides of things. I've read on the forums that a lot of people want a swampy area in the game, well there you go. These states can also offer a lot of new (mutated) wildlife to the Fallout Universe and give a Point Lookout(y) feel aswell.
Couldn't really find many good pictures of Connecticut or Rhode Island, but I'm pretty sure they're cool places to see. They could be the more... 'civilized' areas of the game?
Since Maine seems a little too far for it to be in the actual game, maybe we could visit it in a DLC? Just some thoughts...
Fallout 4 has to be a sequal to Fallout 3 and the only way of me seeing that is if it takes place in the Commonwealth, because it was talked about occasionally in Fallout 3 and three of it's DLC's. A few characters in Fallout 3 are from the Commonwealth, and a sort-of main character from Fallout 3 even traveled to the Commonwealth!
Some factions in Fallout 4 could be:
The Institute - This faction's HQ is the Institute of Technology. This faction is advanced in cutting-edge technology. Powerful weapons and armor, and have Androids on their side as well, making them powerful foes or friends.
Synth Retention Bureau - This faction is a part of the Institute. This faction's main priority is to capture and return escaped androids that escape from the Institute. I can see this being a major role in the Main quest. Maybe an escaped android gives the Player Character a holodisc that contains vital information that the Institute needs to... maybe take over the entire Commonwealth wasteland? Or maybe something bigger?
The Railroad - This faction only has one goal - to save escaped androids from the Institute. They are direct enemies of the Institute. This faction can also be highly advanced, since they take in so many androids they already have strong soldiers cause of them... maybe the Railroad isn't so "good" as they seem... maybe they're helping these androids for something more sinister... maybe they are re-programming them to become mass murderers or something.
Brotherhood of Steel - Obviously we know who they are. This BoS Faction will be part of the Lyon's BoS, because... well, they traveled across the country for D.C.'s technology, why not travel to the Commonwealth (where the most advanced technology is)?
Enclave - We know who they are as well. I don't know much of the Enclave, I know they were the American Government (or something of the sort), so I'm guessing they could've already have been in New York already... so those building's in the outskirts of New York could be an Enclave HQ or something.
If I can think of anything else to put in my suggestion, I will. Thanks for reading.