They could possibly reward a sort of battle high which is only rewarded to those who complete objectives within the group and it would increase pvp xp and tomes at the end of the round. Dying removes a stack so sticking with the group will be a natural way to maintain a high amount of stacks for maximum rewards at the end of the game. This is the only class neutral objective I could think of that promotes team play, no afk, and no solo roaming. Problem is it might promote a fear of scouting new objective and/or zerging.