After a rumour I heard recently. I think we've got as good a chance as ever of Devastator being the next one they bring out.
With his involvement in the movie Superion would be as likely as any too.
Have to be honest, if a combiner/gestalt was obscure back in G1 then there is about a 0% chance of it ever being re-issued.
They are only going to re-issue the popular characters. We may have had some luck in recent years with the Encore line in particular giving us some nice surprises but we have to be realistic. There's no point in raising your hopes for something that is never going to happen.
My most wanted G1 reissue is Shockwave. He's a leader class character and I know I'll still be extremely lucky if he ever gets brought out again despite being in the new movie and being so popular.
Sorry, not trying to pour cold water on people's hopes but beyond Menasor, Superion, Technobots, Protectobots I think we'd be more than incredibly lucky.