My suggestion for Heavensward is a continuation for Manderville side quest. I noticed Heavensward have some FFIV references like Dark Knight, Wind-up Kain, Cecil's crown, so, how about put some FFIV characters in Manderviile side quest, like the Four Fiends (a trial with then) and put Palon and Porom to help Manderville and friends discovering Heavensward or something like that, I think these two are perfect for this quest line.

I would like see Manderville again and I think this sidequest is perfect to put characters from another Final Fantasy. Ah! and dub Rubicante with Norio Wakamoto!