I don't think it'd be too far beyond the realm of possibility that they'd make the CT questline required for future story developments. I mean, most people do it anyway. Not so much for G'raha, but because Cid and the rest of Garlond Ironworks are major players in the main storyline (and will most likely continue to be) and it'd be weird to see them again without referencing anything that happened in CT. The CT quests mention things about the main story (such as you felling primals and being the Warrior of Light), so why can't it be done the other way around?

Though it is interesting to note that the main storyline does mention Frontlines even if you have never participated or even unlocked it. Frontlines are even pretty important to the plot, what with the Carteneau Flats being the location of Omega... This leads me to believe that participating in Frontlines at least once will be a requirement for story progression. They made us to a bunch of guildhests and the story has us revisiting old dungeons (Qarn) so why not Frontlines and CT?

I loved the post above about Eureka being an underground storehouse for Allagan Weapons... As in the Diamond, Ruby, etc weapons. Though I can't imagine us fighting all of them one-on-one (or 8-on-one) in duties. Considering how they roam around in FF7, though, I can see them being FATE bosses or just being eliminated in the storyline.