I think it's going to be thin at first. Square Enix has 300 people working on FFXIV full time right now, and can't spare any manpower for FFXI beyond what they already have. The dev time is spending every moment they have just preparing the initial launch of SoA, and while they likely have the whole expansion storyline planned out, they won't get started on the next infusion of content until after they've fixed all the major bugs from SoA launch.
I fully expect a WotG style, long, drawn out story expansion. That's hella better than the weak "scenario add ons" that they followed up with. At least with the Abyssea mini expansions, we got a bunch of goodies such as level 99 and whole new armor sets to go with it. (And a surprisingly interesting mini storyline, too.)
Despite WotG's long drawn out story, I was so busy with other content when it came out that I didn't actually get around to finishing it until last month. Just rounded out the Bastok quest line last night, and my static is on track to finish up the Sandy storylines before SoA is released. (We did the Windy storylines along with the main trunk missions.)