Tera does not have any good PvP content, Killian is dead and mystel is a pve server and GvG system is bugged. There are only a few battlegrounds, which barely pop.
By removal of FWC there is no other bg for mass pvp, except CS, which is with equalized geared. Tera SEA has all of the old battlegrounds, including FWC and CS, which are solo queue and Premade Queue as a raid. I have attached screenshots from the SEA. This will make players come back and enjoy the pvp content.. We know that you cannot bring alliances back to the server. At Least you can get these Battlegrounds back.
If Tera SEA can make it possible, then gameforge should be able to do it as well. Most of the players have already quitted after the last best Patch 80. If you don't take any measurements and keep your attitude and politics of not considering players' needs and expectations like this, you will lose more players.