Galenos. Tuathá de Galenos, whose name is odd, since it means 'the people of the god Galenos'. I guess the implication is that he's Celtic or something, which I thought would be covered with Caerleon. He hasn't even spoken a single word since it was retconned (originally he spoke Guyot's name, but that was altered in the edited versions of the older volumes).
He breathes fire. We already know at least one Zoanoid could merge with the landscape. Maybe he becomes a volcano? A giant, humanoid, ambulatory volcano. How does one stop a volcano? To quote a geologist: "You don't." Another theory I had was that maybe his power involves radioactivity. Guyot's power was gravity, Prukshutal's was electromagnetism, I am guessing their force fields (Especially what Jabir and Clumeggnigg are doing now) could involve the strong nuclear force... so why not the weak nuclear force? Yentsui's ability seemed to be quantum-mechanical in nature (Contrasting nicely with Waferdanos' intensely organic powers), and Caerleon's 'casting threedimensional shadows from a pocket dimension' power reminded me of the angel Leliel from 'Evangelion' - another quantum effect, in this case a Dirac Sea.
As for Barcas: Wasn't it suggested during the battle in Arizona that once he takes on his battleform, he can't switch back? If that is true, it's more like a conversion. His power is psychic - the mind. He is Chronos' most powerful telepath even in human form. So... perhaps, when he changes, he becomes pure psychic energy. No baroque organic warrior form, but only a mass of structured energy. Like the final form of Onslaught.