Not really sure why the Sonics left to begin with, although I'm sure it was for good reason. Seattle lost their beloved team in 2008 when they became the Oklahoma City Thunder. Ray Allen seems pretty fixated on the idea that everyone loved going to Seattle and playing there, yet I really don't think anyone did. It's completely out of the way for most teams, making plane rides longer, making road trips more of a hassle, so I completely disagree with that point.
The other big hurdle would be if you add Seattle back into the mix of teams, then the western conference will have more teams than the eastern conference, leading to the NBA having to add another whole franchise into the equation. I doubt Adam Silver, commissioner of the NBA, would want to begin that process. Sounds like a whole bunch of more problems when he's already got enough on his plate to begin with.
Seattle does have great fans, the 12th man for Seahawk games definitely proves that. I'm sure they'd love to have the Sonics back to give them something to cheer about in the spring months, but I don't think it's a realistic option at this point. Sorry Sonic fans, it might be awhile until your city gets its team back, that's IF your city ever gets its team back.