I'd love to see Quimath, the Kyton lord of Jangling Hiter, get some more press [as well as that Ancient Baatorian he unknowingly has chained up in his tower...]
I'd also like to see Apomps the Triple Aspected get some airtime, being as how the 3e FF did their best to ignore him [if whoever wrote the entry was aware of it].
Then there's, well... all of the unique yugoloths: Mydianchlarus, Anthraxus, Bubonix, Cholerix, Typhus, Taba, Xenghara, and Charon. And then the Keeper of the Tower Arcane, Helekanalaith [who has a single quote and a listing of his title, and that's it ever], and his predecessor Larsdana Ap Neut [who likewise has a few lines on her in D&D ever]. I've made something of a pet project with the history and details of those latter two, just because of their role and the fact that they've never gotten real airtime in the published material.
And of course, Daru Ib Shamiq. Cannot forget him.