I think you can appease hardcore players -- but I do not think you can appease hardcore players who have a narrow scope of what they want from a game (if they even know what they want from a game). The latter is the type of player who comes to the forums complaining about this or that or them.

I think any hardcore player ('hardcore' in the truest sense) will always find new goals to set themselves within the game, or are hardcore because they enjoy the game and are not easily dissuaded from it as the 'other' type of hardcore players are. Clearing every dungeon and amassing an armory of all the best equipment (to me) does not mean that person is hardcore -- this type of player generally does not enjoy the process that went behind the acquisition of the gear and will eventually quit to do the same in the next MMO release.

True hardcore players are the people online six, twelve, eighteen months later still tearing the content a new one and contributing positively to the community.