Where ever Vault-Tec decided to position Vault 69. THAT would be interesting to search the remains of!
I'd like to see lots of interesting vaults, the experiments I learnt about during my play-through were interesting and unnerving, and I'd like more of that. As for setting, if vaults don't already have a designated position and they can throw them anywhere, I'd like something similar to Fallout 3. Lots of wasteland, but also a ruined city or two. I'm not familiar with American geography, so I wouldn't know where exactly the best place would be...
To be honest, the most interesting cities would probably be Washington, Las Vegas or New York, two of which have been done and the other wouldn't have enough wasteland for my liking. Unless there's lots of open areas around New York? I'm not sure, any Americans confirm that for me?