Basically a CTF style game mode with 5v5 or more players. Each team would spawn on opposite side of a corridor style map (think TF2 ctf if you have ever played that) and each team would have an Orb of Power as their flag. Maps could have vertical levels via jump pads and random items that give utilities such as a portal gun or shield.
Offensive: The orb would give a temporary bonus (improved crit chance, speed, etc.) when captured.
Defensive: Players can use supply to build structures and turrets, can make mazes similar to a tower defense game.
This is just a few basic ideas, please comment to help improve and maybe Anet will add this someday!