The thing to keep in mind with SG is that it is extremely unlikely Hasbro will make a new mold for an SG release. Maybe they would debut a mold, but probably not develop a new one.
With that in mind, you have to ask yourself, what molds are currently (last 2-3ish years) available, or leaked/announced/planned? What characters have been done, and what characters have yet to be done? Additionally, what characters might have a dual purpose? When SG isn’t doing "opposite" or alternate colors, sometimes it's throwing an obscure deco on a character (SG Slicer's deco was used for SG Wheeljack, but was an homage to Slicer, SG Flamewar is just regular Flamewar's deco).
I think SG Thundercracker would be a popular figure. We also have an upcoming Skyquake; an SG version could have a G1 deco or Dai Atlas or Machine Wars Starscream, all of which are decos that someone at Hasbro might want to sneak into SG. SG Tarn might be interesting, though the mostly obvious thing would be to swap purple for red. The Insecticons have current releases which might be good for SG.
Until we get some more leaks or reveals, all we can do is speculate, but there are ways to make educated guesses.