Germany would be a great place to do it. It is very easy and fairly cheap to get there, there is a strong rock fans community and people like to attend live gigs. Furthermore, live, this genre of music would attract people. I am convinced that you could easily get 1,500 people in Germany. Sweden, considering the number of good bands coming from this country is another alternative. The problem would be to fly the bands over. But you would not need to get 20 bands. As it was stated earlier, 2 strong headliners plus 8 good bands and it could be viable over 2 days.

For the rest, I find JMac's comment to be extremely relevant.