Since we get to wait a good year or so for the next FO and much longer for a FO4, why not throw out ideas for future content, and maybe one or two will stick with the developers...

Mine would be you start in the middle of a new war. Not a war with another country, but between two (or more) major factions trying to take control of what's left of the U.S. We've seen and heard of the Enclave and the Commonwealth. Perhaps fast forward 25-30 years into the future and the Enclave has rebounded in the West and is trying again to reclaim the East, where the Commonwealth has begun to spread out. The two have massive armies and are focused on their overall objective -- retaking the U.S. Neither wants to budge, and so a war ensues. You begin as a neutral party, and can remain so throughout the game, or side with one side or the other (or insert one or two other factions).

The possibilities are endless...

What are you talking about, the commonwealth? It's stated multiple times in F3 that the commonwealth is blasted nothing except for the closed environment of "The Institute."

I think everyone was hoping for some Commonwealth DLC, but that looks to not be happening. Maybe a surprise expansion? Or perhaps part of Fallout 4.

Well, despite what Zimmer had to say, there has to be something left. I find it hard to believe that all of Boston and the surrounding area have been erased from the map but the Institute remains. It would make more sense that the Institute were absolutely flattened as the main target for the Boston area Zimmer seemed rather full of himself (aside from being kind of a jerk) and I don't think that it would be out of the question that his idea of a "blasted nothing" isn't really that much worse than the CW. As of now, we are relying only on his word. Unless his word were corroborated by someone that isn't from the Institute, or at least not as arrogant as Zimmer, then I think we can say that there really isn't much left of the Commonwealth.

Now my idea... The Institute and the descendants of military and guard units (not like the BOS) in the Boston area find themselves in a war. The PC is charged by a party not involved in the fight to retrieve something from the Institute or be killed on return. The PC could side with the Institute, the Army of the Commonwealth, or go it alone. Unlike F3, both sides have good aspects to siding with them and some really terrible ones. Also, there would be true good, neutral, and evil paths, not just a good, neutral, or evil way to finish the main quest. I have it all planned out, but I doubt anyone wants to see the 14 pages of text (I got bored).