I think this game should look to games like World of Warcraft in what they do with their instanced battle games. Make it easy like a load in button you can place on your hotbar. I think you would be able to sell it to more people if you had some kind of instant action fun. Something where you can spawn your items, or select a preloaded class type, and just battle without it effecting your main world character.
Something similar to this that i've seen was the oroboros tournament server. I am suggesting however that the dev's set up some kind of capture the control points game, so that every player upon entering the game world can immediately experience the best parts of the game, ie, large scale warfare.
Set up a 50 per team red vs blue game, and a red vs blue vs gold game, let the people teleport to the battlegrounds mode and people will love it. Then more people will play the game for the minigame battlegrounds mode, and subsequently more people will join in on the castle building in the mainland. Then you will have satisfied a greater diversity of players from casual to hardcore.