First blog, and I will be blunt so people don't see a lengthy wall of text. All mmos are failing today except WoW because they concentrate too much on PVE content. PVE content in its current incarnation is a waste of time and resources. Its also the leading cost of high budget mmos.
Its been said, 1st time mmo developers underestimate the problems of mmo development and why it takes so long. Content/story based games have difficulty because a player can play through the content of a single player game in 48 hours or less. Its fine for a game they paid $50 for, but for mmos the revenue comes for attracting them for months. Its a constant uphill failing battle to try and offer compelling content for your users month after month. This is practically creating 5 single player games a month.
My solution is simple, and I will break it down to execution.
1st you want look at whats been successful at capturing the players attention for long periods of playtime yet offer very little in the form of effort. This aspect can be seen in competitive multiplayer games like Counter Strike and Starcraft. When competing against other players to achieve a goal players tend to be interested for longer periods of time. This is evident in mmos with games like Lineage II and Ragnarok Online maintaining its popularity through guild owned castles and sieges. So what you want to do is make a PVP-centric game.
2nd is you want to expand on this. Castle Sieges are a good start, but its something that only happens on a weekly basis and doesn't really offer the variety people want in a game. I think good avenues of exploration for PVP based competition are expanding trade in the game. Political systems. Town Raids and protection systems, Mini-games such as card games and racing. All of which has the possiblity for addition by freeing production time and costs by knixing generic PVE content.
3rd. Although I have been critical of PVE Content. There hasn't been an mmo made that has had success without having to knock a few AI heads. However, what you shouldn't do is waste the time with static monsters and generic quests its wasteful and time consuming. You should concentrate on scripted events such as protecting villages from monster attacks, getting a princess back from a raid boss who kidnapped here, and protecting trade caravans from brigands as they travel around. Maybe sometimes encounter that hungry bear who wonders the forest.