Word on the street is Inferno and (another) Starscream are coming next. There's also rumor of an ehobby exclusive Artfire, which is super exciting considering he's the rarest, most valuable G1 Transformers ever made. He's also an awesome repaint of Inferno!
It's been a strong rumor for a long time now, but there's no official word on it. Same with Soundwave. Both are SUPPOSED to be coming, but Takara hasn't officially announced them and there is no release date scheduled.