I think august is a good month. It's more expensive but it's easier to have days off at work to go to the gathering (august the 12 & 13th?).
I personaly do prefer big ramps, such Berlin, Mezzano or Lyon.
If we organize the gathering in an outdoor skatepark we are taking a little risk. It would be very annoying to suffer a rainy weekend.
I think that if the meeting is going to take place in Europe, we could vote for the options available.
I would like to include the Skatepark de Lyon in the list. The ramp is quite good (despite de surface is not new) and they have experience and good facilities there.