As a member of the Blackgate server I can with all my heart recommend Kaineng or Tarnished Coast for population and WvW. They both have some awsome wvw guilds running around and both seem to have great communities. TC probably has a bigger pve crowd(probably too big at this point) while Kaineng has the bigger wvw crowd. Both being tier2ish servers so population shouldn’t be a problem on either.