We've already seen Chicago, in Fallout Tactics. That's the Midwestern Brotherhood of Steel's domain, and they've apparently "gone rogue" or something according to Lyons (or another BoS member - I swear I heard it somewhere in the Citadel). I wouldn't read too much into that, given that the Eastern Brotherhood of Steel could also be said to have "gone rogue" by the leadership back in California. All the same, I don't think it'd be too interesting to head back there.
Personally, I like three areas. The Commonwealth and New England would be good option. Seattle would also be cool, and it'd be really interesting to see what the War and two centuries of neglect have done to its environment and infrastructure, plus the Space Needle is pretty much the epitome of the "SCIENCE!" and "The World of Tomorrow!" style that Fallout draws from. Finally, Colorado and the Rockies would also nice. Van Buren is semi-canon as it is, and even if they threw out everything from it, the setting still offers many options, like Denver, Phoenix, Salt Lake City, and the Grand Canyon.
Other interesting areas to see would by the South - I'm thinking New Orleans for some good times :foodndrink: - or Texas. 'Course, I'm a native Texan, so call me biased on that last one. All we've heard is that the oil fields are shot and that Tycho traveled as far as the Gulf before heading back to California.