This is a load of horse crap. Whenever ppl say this, they never really mean it. Generally, this is the translation: "My idea is so awesome no one can possibly disagree with me, so I'd love to hear everyone's thoughts telling me how awesome the idea is, and ppl who disagree are obviously stupid, poorly educated, can't see the vision, or lack creativity." After reading through the thread, I'm pretty convinced the translation is accurate. You are so hyped up on your own idea, so convinced of its greatness, that you are unable to comprehend the possibility that it is not a good idea at all. Really, why bother asking for opinions when you don't want them. The only thing you want is others to job you in order to make you feel more awesome in coming up with the idea. These types of threads I like to call "self-validation" threads. They are threads designed to get others to agree with them, so that the OP can elevate his sense of self-worth and self-importance. Generally the OP takes a stance on a topic which he feels will garner the most agreement. In other words, he picks a bandwagon he thinks a lot of ppl will jump on. If it is an "original" idea (read: creating a new bandwagon) then it increases the risk of disagreement, but also increases the reward due to its originality. Here we have it, a self-validation thread gone horribly horribly wrong. It was a good try though.
Your idea is bad, at least for MMORPGs. For one, its not even an original idea. Crossovers have existed for decades, in all parts of media/art/entertainment. Everything from All-stars games, to spin-offs, to music mashups, to collaborations, all represent the same idea: bring a lot of popular things together into one product. I'm almost certain this idea gets tossed out with every product a franchise has ever though of creating. Its a basic idea, easy fanservice, and requires the least amount of effort. You can copy/paste designs, with a little updating/consolidation, re-use certain animations, spin-off existing storylines, all while raking in tons of money because nostalgia will cause ppl to buy just about anything. And in most cases thats all that needs to happen, SE could care less if you burn it 5 days later..they've already locked in profits from your purchase. When it comes to MMORPGs though, SE can't afford you to quit playing...they need you to keep paying for the product. Quite frankly, nostalgia only lasts but so long before an individual recognizes the product for what it is: empty, uninspired, blatant fanservice. At that point you lose all will to play, because there really isn't anything there.
Let me explain a few flaws..
1) Lack of a Persistent Universe: The final fantasy series does not have a persistent universe. This is the biggest roadblock. Because it lacks a persistent universe, there is no thread connecting the various games to each other. None at all. This means a thread will have to be *forced* into the plot in order to connect Midgar to Balamb Garden to Luca. Due to the sheer difference in universes, technologies, clothing styles, city design, etc...anything you do to connect them will be very obviously forced. And there is nothing more disconcerting, and distasteful, in a storyline than a forced universe. A universe should feel natural, not contrived. A contrived universe is an ever-present reminder to the gamer that they are playing a game and not participating in a pseudo-reality. It breaks the fourth wall and leaves it in pieces.
2) Lack of a coherent or original storyline: Related to the first problem is the 2nd problem, the storyline. There is really no good way to go about it, because there is no common denominator. It always boils down to some arbitrary reason worlds are forced together, usually due to some time-space anomaly. Sonic Generations, Kingdom Hearts, (insert other game here), they all use the same storyline with a different villain. Its old. Its shallow. Its not even creative. It can't be, because there just isn't any other logical rationalization for mashing a bunch of worlds together that existed in separate universes. Efforts to be creative usually end up as incoherent and convoluted, precisely because the topic (time-space anomalies) is not something easily understood even by the most accomplished of physicists. Now you're asking everyday gamers to understand it? Lol.
3) Lack of consistency: I'll use kingdom hearts as a perfect example of this point. For those of us who have played through the final fantasy series, we know how powerful some of the characters are. If Cloud shows up in my FFXIV, there is no reason why he can't go destroy (insert primal, villain, army here)...or why (insert black mage here) doesn't just drop a meteor wherever the hell he/she pleases. These crossovers have horrible consistency when it comes to power levels, precisely because they were originally balanced for single player games in isolated universes. Take Kingdom Hearts. Explain to me how a kid who's been swinging an oversized key for less than a year is able to handle both Cloud and Squall in the Coliseum. There's no way. Squall could one-shot him with any given limit break, and Cloud could lay him out a few times over. Tell me how a few swings from a key can put Squall down, when Squall can tank giant explosions from ultima weapon? You may think this is petty, but these inconsistencies can make die-hard fans of certain characters nerd-rage like you wouldn't believe. Its stupid for a boy who's balls have yet to drop, who hasn't known war, and has yet to see puberty, can fight two veterans who saved their respective planets on equal ground. Just. A. Joke. Explaining this in a universe also requires contrived reasons, further destroying the mind's ability to suspend disbelief.
All in all its a bad idea. I could go on, but it just doesn't work for MMORPGs. It can work for single player games, sure, as they dont need to be fun very long. But for a game you want ppl to pay for month after month? Just no. Nostalgia isn't that powerful.