Well we already have a Midgardsormr in FFXIV and we've already had multiple battles with it.

We might get something like Jenova someday but it would have to make sense in the FFXIV lore, similar to what they did with Anima for this game. They could have probably even named that Jenova and been fine.

I think just bringing Sephiroth into this game would be lame. If they had a big role that did them justice instead of making new main characters like Emet, Zenos and Gaius, it would be lame. If they had a small camero role, it would devalue what Sephiroth is. So I doubt it'll happen.