This isn't something I ever expect to happen, but I just keep thinking how awesome it would be if there was integration between Fallout: Shelter and Fallout 4. Even if it was just a little easter egg of a vault with your vault number on it. You wouldn't even necessarily need to be able to open it.
That kind of minor integration I think would be plausible. I don't expect it, but I think it's a possibility. Now here's the idle dream part: What would be even cooler than an easter egg would be full integration, where you could visit your vault and stock up on things, and it would have all your dwellers there and everything.
But as I say, I don't expect that to ever happen. If it would even be possible. Still, though, any integration between the games would be super cool!