KSTW (under its original KTNT call letters) was actually a CBS affiliate in the fifties and early sixties. As you note, it was a CBS affiliate again for two years in the nineties as a result of a joint deal (CBS wanted to affiliate with KTVT in Dallas/Fort Worth and had to agree to also take on KSTW in order to get the deal from the then-owner of both stations, Gaylord). The likelihood that it will become a CBS station for a third time is probably pretty low since history says that hasn't worked out well for CBS in the past. Notably, if the ratings hadn't been horrible in the nineties, I'm thinking that CBS wouldn't have been as quick to jump back to KIRO as they were.
Whether CBS would be willing to sell KSTW to someone who might want to make it into a CW affiliate is another matter, since I'm not sure how valuable a stand-alone independent station really is to them. While KSTW was once one of the most successful independent stations in the country and was even a regional superstation ("We're entertaining the whole northwest", "KSTWashington"), those days are long gone. Now it's just another "has been" station with a minimal news presence and very little image outside of being a generic CW affiliate -- whatever value there might have been in the station's heritage got blown away during those two years as a CBS affiliate from 1995 to 1997, and nothing that has happened since to recreate that heritage.
I grew up watching that station, but it's pretty clearly that what is left is an empty husk that probably is only worth whatever the stick value is to get into the Seattle/Tacoma market.