I mean, it could be. A lot would have to go wrong in this series, so it's not especially likely that it's his last start as a Ranger, but he is a free agent after the season is over. And he'd be the best free-agent starter out there unless CC Sabathia opts out of his contract.
It was hard to imagine the Rangers without Cliff Lee during the playoffs last year, but they certainly made do (all the way to another AL West title). It would be really hard to imagine the Rangers without C.J. Wilson. The reliever-to-starter gag has been tried before, and it will be tried until the end of baseball, but it will rarely work out as well as it has with Wilson. It got the Rangers at least one pennant, and it could get them two.
Of course, they could always re-sign Wilson. If I were a betting man, that's what I'd guess would happen.