Hmmm… I think a "Wind Waker"-esque, open world Ratchet game could work… if they had planets instead if islands and an ACiT style space exploration system instead of boating… as long as the gameplay itself resembled a normal R&C style, rather than the RPG gameplay you get in Zelda games… Oh, and the artsyle would have to remain consistent with other R&C games; not the super-cutesy style you see in Wind-Waker… and if it needs to be an expansion on QfB, then it probably needs a darker tone to the story/soundtrack/aesthetic, since I think ToD managed to hold up a pirate theme perfectly well, but QfB kind of stumbled on that hurdle…
okay, maybe Wind Waker is a bad example of how I think an open-world Ratchet game should be made; maybe something more along the lines of Jak 3 would be suitable for a Ratchet title?..