Since launch I've thought ranked would be a great addition to the game not only allowing new players to be put into a environment where they can grow and not be deleted by veterans of the series that understand the combat more thoroughly. But to provide a more challenging environment for players that wish to be extremely competitive. Aside from that simply having ranked in the game pulls in a competitive player base that wants to push the limit of the combat and show just how amazingly strategic(As in the captain mode) or purely melee/ranged oriented a mode such as skirmish would be!
PS: I'd love to see a full fledged arena game mode just as how it is in singleplayer!