I predict that we will see Sin from ff10 as an 8-man or 24-man raid with it's own storyline, which would work since ff10 always emphasized how colossal sin is.

And we'll also have Seymour Guado as a trial

I think this will happen because the fanfests have given out ff10 glamours and pets, as well as the implementation of swimming, something only seen in ff10. (Although I haven't played 11, so I wouldn't know if that has swimming or not)