As far as domestic service goes, I think SAT could use nonstop service to JFK & BOS(JetBlue E-190), MIA (American MD-80), and SEA (Alaska and/or American B737, MD-80, respectively). YYZ on Air Canada Jazz (CRJ-900) would be nice as well.
Yet, given SA's close ties to Mexico, I would still love to see more nonstop service to Mexico. United, (which currently has a weak presence in Mexico and the southern U.S.), could use SAT as a gateway to Mexico and provide nonstop regional jet service to several mexican cities such as Leon/Gunajuato, Torreon, Chihuahua, Puerta Vallarta, Mazatlan, etc. United already serves all five of its hubs from SAT (DEN, ORD, LAX, SFO, IAD).**
**United attempted to operate a "hublet" or "minihub" at SAT in 2006 which included nonstop regional jet service to MSY, ABQ, COS, OKC, TUL, MCI, and OMA, but all flights were soon discontinued due to low loads/yields. I believe, that with the proper connection opportunities, cities like PDX, SEA, JFK, MIA, and COS, could support nonstop service to SAT with connecting service to the Mexican cities mentioned above.
Perhaps someday...