I love GW2, but at the moment group combat feels like everyone is bashing on a mob independently, rather than working together as a team. I think with some small changes, GW2 group combat could become more strategic, challenging, and fun.
The key concept of teamwork is communication. GW2 has the concept of combos, which is great, but I think they seem to happen more by accident than planning.
I would like to suggest having the ability to communicate your skill (via speech bubble and a short pause) before you execute it to enable other players to get ready with a follow-up combo skill.
In addition to this, it would be nice in the future to build upon the combos idea to create combo ‘chains’ that give greater damage bonuses the more moves you can chain together with other players.