The one downside I distinctly remember from first time was the opening round. Most of the people who signed up didn't show, and every single first round server had all players advance, or at least nearly everybody did. A backup plan for that scenario would be nice, like adjusting the number of servers used in the first round.
Afterwards, the only issue I had was the ban list, or lack thereof. You wouldn't need to make it the same as ESEA, but take some suggestions from it at least.