All the crossover events so far have had back story written into the lore. I think it would be extremely difficult to make up story for so many different characters from so many different games with different backgrounds. Though fighting them would be really cool, I don't see this as likely. I mean, who doesn't want to take their FFXIV character and fight someone like Sephiroth with it?! There weren't many good choices to represent Eorzea, so I think Y'Shtola was an okay pick. I would have liked Gaius myself. Not to be too spoiler-y, but most of the NPCs in the storyline especially in the Scions seem so useless as you go about the game and do all the work for them. Thancred probably would have been the only other choice, but I don't think you could make an original character with his abilities.

What else I would like to see for crossovers:
- Turks from Final Fantasy VII
- Espers from Final Fantasy VI
- Lavos from Chrono Trigger. Or maybe Magus. Or maybe some cool futuristic thing with Robo and Lucca.
- Anything from Final Fantasy Tactics
- Anything from Brave Fencer Musashi
- Anything from Vagrant Story