Let me make this clear that I'm saying this as someone who likes Dissidia a fair amount, be it vanilla or duodecim...
Dissidia is an awful game for a Tournament like this. A few important factors to consider:
-Poor Spectator Sport. All the games above can be viewed by people NOT playing the game, cause they're on TVs. Dissidia...we could hook it to a PS3, but even then, that's a hassle.
-To add to that, Dissidia fights can take a long time. Generally speaking, all the other fighting games can have a forced Timer on, and then games like Brawl just eventually end anyway, cause the way damage racks up, someone's dying. Dissidia? Fights can go on for quite some time with the two sides just trading a lot of blows with Brave damage, never getting that crucial HP Hit in. Oh sure, for some characters that's not likely to happen (eg WoL or Zidane, where a successful Brave hit often translates to instant HP damage), but other characters (like, say, Emperor) that's not the case, such that if we get two characters like the latter going at it, the fight can last as long as 10 minutes. I've heard of Dissidia fights going on for nearly an hour in some case; no, I don't suspect that'll be the case, but a fighting being 5-10 minutes long is not unreasonable (if still a bit extreme.) This is just not acceptable, really...
-the Character raising aspect. Granted, we could either use pre-set Arcade Mode characters (which...wouldn't be fair to people maining some characters who have poor sets), or use characters based on what others have done, but then we have to remember that...
-Dissidia is *NOT* newbee friendly at all. All the other games are pretty conventional in "Hit things til they run out of health." Dissidia has a pretty high learning curve, which unless you play a decent amount of the single player campaign, you're unlikely to adapt to the game's basics well
I will grant Dissidia has really good Tutorials, but it takes time to go through them, since the game teaches things to you one at a time, and lets you learn from experience, such not to overwhelm you; again, this just further proves "Dissidia has a firm learning curve."
We've discussed this in the past, and Dissidia just isn't feasible as a tournament game. From what I understand, Tournaments in Dissidia are usually held over the course of days, due to its unique nature and how the game has to be approached in a different manner, as opposed to many fighting game tourneys which are done in a manner of hours (the DL is much smaller than most tourneys, of course, so yeah)
Its...really not much different than proposing COMPETITIVE POKEMON TOURNAMENT in the DL, except Pokemon matches are a lot faster...just the main difference is now raising Pokemon ACTUALLY MATTERS (Dissidia, you can easily shift a few skills, equips, etc. around to your liking if you have a character with everything (significant) maxed, so the "borrow someone else's game" factor works. Pokemon...turning a Special Based Mewtwo into a Physical Based Mewtwo just kind of doesn't work...at all...let alone on the fly)
Due note I am not saying we shouldn't play Dissidia as a CASUAL game at the con; I'd be fine with that. Just as a tournament game, its not something I'd get behind. Again, I say this as someone who likes the game quite a bit.