Alright, I took a look at your tournament and I'll do my best to point some things out.
Hopefully I don't repeat too much of what Kirbywizard and Shadowfire said.
When it comes to making a large, successful, THEMED tournament, you want to organize your OP as much as possible and make it easy to read for potential contestants. Making it aesthetically pleasing is always a plus as well.
Your OP is ok in both regards, but it could be better. It's not really a huge issue, but it's something to think about. Take a look at my Gym Battle tournament to get a idea of what a good OP looks like
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Your theme is original and decently well thought out, which is good to see. But some parts instantly turned me away from wanting to join, and I'm sure it did for a lot of other people as well.
People don't come to OTL to fight computer players, and from what I saw, CPU's play a large part. Correct me if I'm wrong. I know there are some human players as well, a few of the bosses I believe, which is good.
Also, NO one is going to want to take the time to send you replays of every match they do to confirm that they did the match and didn't cheat. I know it's probably the only way to be sure they did the match correctly, but that's why CPU's are never a good idea in an online tournament. That and they're boring to fight.
Items CAN be used in online tournaments, but try to avoid using them if you want to attract a large amount of people. Items are best used in smaller tournaments or not at all.
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A prize, if you can afford to offer one, is always a good way to attract some decent players. I've offered a prize for nearly all of my tournaments, even if it was a small prize, and they were all relatively successful. Tournaments do not NEED a prize to be successful however, but it can help.
Themed tournaments will either fail miserably or be decently popular. I've only ever done a Low tier tournament, and my current Gym Battle tournament. I wasn't sure my Gm tourney would even get 32 people, and yet people kept on joining. I even had people wanting to join after I had 64 people.
This isn't really a pointer, but I feel that one of the main reasons that people joined my tournaments is because I'n decently well known, especially in OTL. I've been participating in online tournaments since Summer 2008 and hosting them since last December. I know that sounds stupid, but it's true. You'll see an OTL regular make a tournament, and they'll get 32 people in a couple days no problem. I've seen it happen multiple times.
Getting yourself well known kind of just takes time. Also, sometimes starting with a simple non-themed tournament when you're new to hosting tournaments is a good idea, but you've hosted two already as far as I know.
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I think that's all I have for now. Just keep at it and hope for the best. If you have any more questions then feel free to ask me.