I didn't join this year and had other things going on at the time, so I never really looked at the final lists either.

That said, I don't think the level lists are the thing to focus on if the question is how this event could have been more successful. While compiling them is a good tradition, actually they aren't very important. It's nice to have lists of levels to run, but between mutators and JJ2+'s server admin commands they're guidelines at best (or should be).

The level lists may be able to create hype among people who make levels that have a chance of getting into them, but otherwise, meh. If the goal is to drum up some enthusiasm prior to and during the event, I think it would be more useful to focus on fun things everyone can enjoy. Interviews, retrospectives, maybe a small tournament or something. You don't even have to convince CliffyB or Arjan to answer questions, it would be fun to take a look at the stories behind famous levels or community figures or events as well. Maybe write a mutator that allows Jazz2Online visitors to influence the game somehow. Anything that actually offers people something fun to do that's not just playing the same levels they've been playing for 20 years, but in a slightly more busy server. That's an important part of the event, and I wouldn't advocate getting rid of it, but clearly it's not enough on its own.

Or don't change the formula and call it quits after 20 editions. It wouldn't be unreasonable. But I think we can still find ways to make it fun, it would just require doing a little more than just compiling a list of levels and hosting it for a few days.