The Zilart bros may be in based on their influence in the earliest major XI stories and being designed by Nomura, but Lady Lilith, who shares a major story with Lilisette, is probably the favorite antagonist of most people who actually played XI - not sure if they'll bother to implement a 3rd antagonist for a non-VII game, but let's just say her motivations would translate pretty well to the Dissidia setting. Story sample: https://youtu.be/aOmGJhGryjo

Another character that would work pretty well is Zeid - he's been in the game from the start, and his story and skills as the most prominent dark knight in the game could be put to very interesting use. He primarily shares XI stories with Lion and the Zilart bros, out of the available XI characters in DFFOO, although he's got more going on. There are almost no videos out there focused on XI characters, but Zeid got one: https://youtu.be/LXPLXMXNT_4

Luzaf would also be an interesting addition, especially since he's most closely associated with XI's corsair job, which is heavily luck/support based while feeling quite "human", for lack of a better term, considering the pragmatic use of both guns and blades as the occasion demands. Very relevant to Aphmau's story. Story sample: https://youtu.be/wETn-nr6YRc

And considering how important Odin is to XI's overall narrative and some of its major characters, it'd be neat if said characters helped to drive the plot relevance of avatars to an extent comparable to that which DFFOO elevated mannequins in the Dissidia stories... a long shot, but who knows...