We only got time for one Seeker, in ROTF: the eternal glory of Jetfire. There was a chance of one more (Ransack, in a fight that only occured in the novelization), but unfortunately they’ve all been relegated to the comics. Still, there might be chance for more Seekers to appear in the next films. I think there’s scope for Ramjet (in his UNIVERSE version, as a foe of Vector Prime), Sunstorm (a special solar fusion reactor for energon), Dirge (a fear-inducer like the Scarecrow), and maybe even Slipstream (a female Decepticon would be something new). Keeping in mind that these are ancient Cybertronians and not Starscream’s brigade of aerial fighters, it could possibly go down well.