Chiming in, this has the potential to be a really great idea.
I think that we should keep it somewhat focused on ENworld instead of a wholly make-believe scenario. At least have some standard EN tropes sprinkled throughout like:
- An elderly, well-mannered grandmother who won't take no guff (w/ purse of smiting +1)
- Furry feline pirates!
- Hot italian beef!
- Monstrous Squirrels named Nutkin
- Etc. Although, the cheese factor can be lowered.

I definitely will need to change my plans for next year. After having grown up in Milwaukee and getting to go every year, I do miss it. DragonCon just isn't the same.
Also, I'd prefer not to have some 40 person battle royale or a stylized tournament. Maybe it's just me, but plenty of these are already in existence. I prefer the many group format where everyone meets up at the end (w/o the final battle die-rolled out).
It would also be cool, if every group was playing a different part of one overall story. This may be tough to plan beforehand, though. Maybe a story could be developed where the number of players and groups doesn't need to be known beforehand. That way if attendance at such an event grows or shrinks, the event still can be run as is. (all you need is enough DM's to cover every group).
I would suggest a single time event, too. This gives everyone plenty of notice on when and where to be. Also, one evening (however long) won't take time away from everything else at the Con.