I haven't spent a lot of time speculating on what other figures might be in the line but just off the top of my head I'd throw these out there:
That's only 13 but I think after those the figure choices start becoming a toss-up. Those characters are all relatively core guys to the Joe universe. I also think some good character choices would be Tele-Vipers, Dial-Tone, Flash, Outback, Recondo, Shipwreck, Rock-N-Roll, Clutch, Wild Bill, the original Dreadnoks (Buzzer, Ripper and Torch), Scrap Iron, Tomax/Xamot and Dr. Mindbender.
Hasbro has a really nice sculpt of a HUMVEE now but how cool would it be to see a classic VAMP with Clutch? Maybe a 2-pack with a VAMP vs Stinger with Clutch and a Cobra Driver.