The thing is,
Yoshi already stated ages ago that they're keeping to a set of rules when it comes to adding characters/antagonists from past FF games in fanservice appearances in FFXIV - things like Cloud of Darkness and X-Death kind of get around it by not even technically being actual characters anyway and just forces of nature (X-Death was evil sealed in a tree, and Cloud of Darkness is... the personification of darkness, nothing more).
So hence you're not going to be seeing the likes of Kefka or Cloud in a fanservice fight, because of Yoshi's following statement:
Accordingly, that counts most FF antagonists out because, from FFXIV's perspective, they're 'already dead' or their 'role has ended' in their FF title (like Kefka, Sephiroth etc). Of course nothing is stopping SE from violating their own rules, but I actually respect and commend Yoshi for sticking to his guns in this regard, instead of bowing to fan pressure for pointless fights that go against this very principle.