In an interview with Sega developers, Noriyoshi Oba of Overworks (the developers behind Skies of Arcadia) stated that a sequel to the widely-acclaimed game is being planned.

Here's the quote from the Kikizo games interview:

You seem like a very happy guy, why such a violent game like Shinobi? And is there going to be a Skies of Arcadia sequel?

Noriyoshi Oba: I don't think it's such a violent game! [laughs] The Skies of Arcadia sequel is in the planning stages at the moment.

Great news for all RPG fans, although we don't know for sure if it's coming to Gamecube. But chances are it will. We'll keep you updated.