I've oftened entertained the thought of being able to experience the story and the many memorable characters of Final Fantasy XI in an offline RPG. It would be nice, if anything, to let non-MMORPG players experience what is probably the most comprehensive and best set of storylines that the series has ever experienced.
So yes, I would be all for an offline adaption that is based around the world of Vana'diel and its incredible set of story archs and lore; even downloadable episodic side stories (see mini-expansions) would be a nice addition to keep building upon the world. However, I'd like to see none of the gameplay carry across.