The only other way to resurrect the Sonics is expansion. There are some important considerations to think through, but I think the case for NBA expansion is stronger than most people realize.
Money
It's got to start here. NBA owners are notoriously penurious as a group, so any expansion would have to end up a financial plus for them. The common argument against expansion is that it "splits the pie" one more time. This is absolutely true, but it also assumes that the size of the pie is a constant. If the new team is a success, the pie will be bigger, period. A new Sonics team would be hugely popular, and draw in new customers to increase the dreaded BRI ("basketball related income").
With a local population of over 600,000, Seattle has a whole bunch of eyeballs and torsos to fill with broadcasts and jerseys. The short-term gains of a relocation might be nice (basically each owner gets a huge relocation fee check from the moving team), but the long-term win of one more successful team far outweighs the short-term pie-slicing problem.
Talent
The league is in a great place right now in terms of talent depth. Some of the worst teams in 2013 - Orlando, Minnesota, Portland, Cleveland, New Orleans and Washington all come to mind - have bright futures, suffering so many losses this season due to injuries and/or deliberate rebuilding. The only truly awful teams are the Bobcats and Suns, who have simply been mismanaged.
The kicker is the 2014 draft, which may be one of the greatest of all time, according to Chad Ford (Insider subscription required). In addition to a ridiculously loaded incoming freshman, a number of top prospects decided to go back to college this year. There could be as many is 6 players in the 2014 draft who would go no lower than #2 overall in the 2013 draft.
Another likely influx of talent comes from across the Pacific. Yao Ming was drafted #1 overall in 2002. Soon after, basketball started gaining in popularity in China. Now, basketball is a legitimate national passion and their basketball academy is growing into a real thing. Kids who grew up in the age of Yao will start to turn 19 (a.k.a. NBA draft eligible) within the next few years, and if you think that a country of 1.3 billion people can't start producing one or two high quality basketball players per year, you are coo-coo-bananas. In case you are worried about height amongst individuals of Asian heritage, I can tell that there are at least as many, if not more 6'2" and above dudes in China than America. Trust me, I'm a geneticist.
Add that to the handful of European and African prospects every year, as well as a growing US population, and you should see a trend towards more and more high quality players entering the league each year.
Balance
Having an odd number of teams is potentially awkward, particularly for playoff seeding. There are a couple ways to combat this problem. The first is to dispense with conference distinctions and simply have the top 16 teams make the playoffs. This has become an increasingly difficult idea to argue against, particularly with situations like this year when Utah won 5 more games than Milwaukee but the Bucks got a playoff spot and the Jazz did not.
There are scheduling issues to consider for the regular season, but nothing so terrible it can't be resolved with a quick computer script. Also remember that Chicago is 200 miles closer to Portland than Memphis is, but Memphis is in the Western Conference. Geography just isn't a serious problem in the modern game.
The other solution is to expand with two teams. Kansas City has a beautiful arena ready to go. San Diego is the 8th most populous US city at 1.3 million, but hasn't had a team since the 80s. Heck, Virginia Beach and Anaheim made pretty strong runs at the Kings this year. Vancouver looms large as a false-start that should have worked. Or maybe it's time to toss a team across the pond. The London Knights playing long home-stands at the O2 arena? I think a few blokes might show up. Basically, I highly doubt the league would have any trouble sparking a bidding war for a second new franchise.
Conclusions
The NBA should expand to Seattle and probably one other city as soon as possible. The owners will make more money in the long term and basketball fans in two more cities will have colors to wear and great players to cheer for. Plus, 32 is a really nice round number: 16 playoff teams, 16 lottery teams.