Considering all the content that I've played thus far, all the subtle (and not so subtle) references translated in, and all the promised content, including Golden Saucer, Crystal Tower, etc (promised years ago, btw),
I'm really amazed that people are surprised, angry, and terrified that Square Enix is putting a character crossover into this game.
Don't forget, Nael Van Darnus was practically Sephiroth in judge armor. All that helmet? Probably hiding his goddamn hair. Nobody was complaining about that. Just the opposite. Your dreaded Sephiroth crossover already happened. If he comes back, well, just look no further than Advent Children for clues as to how they'll implement it into ARR's story.
This game failed once already. And now it's being built on the premise that FFXIV (and the FF series as a whole) should stop taking itself so seriously. And I'm totally OK with that.