Aside from the next part of the living story, an update on the improved daily achievement system and the usual balance changes, ArenaNet will also add a Guild Mission system.
This is really interesting for me as it will make playing in a guild a lot more interesting. There are five kinds of Guild Missions to do which are: Guild Bounty, Guild Trek, Guild Challenge, Guild Rush and Guild Puzzle. Completing these missions will give Guild Merits which can be used to unlock more stuff for the guild.
For more info, check out here: