I hate to sound like a broken record, but:
Bring it back to everyone who wants to go to Nationals, goes!!
Last year at Reliant stadium it was very disappointing sitting at the closing ceremonies and seeing probably more than 60% of the stadium not being filled by people.
I say let everyone compete, add a new field into the mix (maybe Socrates) and everyone will be happy (maybe?)
I think the big deterrent to this is pit area, and people traffic or the fear by FIRST that they won't have enough volunteers to run things.
If this is the case, have a member from each team each day volunteer for FIRST.
I know this more than likely already happens for maybe half of the teams involved, so make it a standard and have it count for points.
As for the pit area, I don't know the layout of The Georgia Dome complex, but if need be, set up a tent outside for additional pits...
It worked for years in Florida didn't it?
