maybe it would be better when both teams would know what classes the other team will play with?
It would kill what little magic there is left in trying to organize such an event.
I mean the initial bracket would be made by organizers in secret, you would sign your team with a certain name for it so later everyone would see the bracket once the first knock-out is done. I recon things would get clear very fast. Not much space for "breaking the rules" as there aren't many and what use would those 4-5 guys have of ruining the whole thing ?
Map for this would need to be specially made, some kind of medium sized arena (so cav can move freely) but not too large so we don't have 2 mins of running before you even see your opponents), with only a few obstacles in it so there is no campfest, just some basic natural cover. You want to field 5 HA's ? Sure why not ? Just don't cry if you can't kill anything. Same as the other team shouldn't cry about inability to take those 5 HA's down. No restrictions means that for example: best out of 7 rounds (quite a good format so its short & efficient) leaves enough time for your team to change their options after the initial round (grab different gear if they are able to equip it, etc) to adjust to the situation. So many possible scenarios that you couldn't list them all right now.