The way I see it making a persistent, online, virtual world game is something that doesn't take a month, a year, but years and many many years afterwards. Developers have proven that they cannot keep up entertaining their player base with the amount of theme park rides they release as well as their quality, so these games revolve around some repetitive grind over and over, which is bad if your end game is based around this.
The themepark formula is basically like this.
Stage 1 - Generate NPC's across a world that you'll have to visit and do quests to get from level 1 to X Stage 2 - At max level get inside the dungeon with a party to kill Y and Z boss to get gear. Stage 3 - Get in a guild to use that gear to get in a bigger dungeon/raid to kill harder boss with 10+ ppl. Stage 4 - Wait for new dungeons and raids to get released. Stage 5 - Do a level increase every year so new players don't feel "behind" Stage 6 - Add some achievement that reward players with titles and mounts for completing game, that you'll use for some time until you get the new ones. Stage 7 - Make some side PvP arena thing where players can kill each other for some website rating, while waiting for next raid schedule, which usually happens in these themeparks 3 times in the week for 3 hours, and that's only if these themepark rides are high quality like WOW.
So, at the end as you can see this type of game cannot grow to something greater, than just release new dungeons/raids and a class here and there. It's basically remains the same thing, but this is where the sandbox type game has the edge over the themepark.
The sandbox formula
Stage 1 - Generate a world with very few NPC's, since players will mostly be filling their roles in the long run. Stage 2 - Create classes/professions that will help players build the world from nothing into something. Stage 3 - Release the game, yes this is pretty much all you need, but the difference will be the game will feature far more classes/professions because of this.
Regardless how less it takes to make a sandbox over themepark, the developer will have far more space over the years to keep making new classes/professions, which would allow players to generate the world with features. Example, you have animal farmer, who's job is to breed and care of animals. You have some bio scientist who can cross breed to design unique species that some tamer could buy and use. Then you have some witch/warlord, who can transform some of these animals into powerful demons. Then you have some jedi (lol, as example) who will unlock the force and players will have to hunt him down for some bounty prize, if he is doing a lot of unwanted damage to the local town.
Bottom line, see where I'm going with this? What's happening is the sandbox MMORPG can only become something greater through time and with players consistently adding conflicts, drama, and unique experiences to players around them. What's happening is everyone starting to be unique individual with unique experience, story, reputation and this allows players to experiment with unlimited possibilities. Where will I end up if I choose to be the bad guy? Will it worth it or is it better to join a guild, do things in a group, safer but also to share the loot with everyone else.
Then after few years instead of developers worrying about releasing new rides that will override the old, they would have a sandbox world with 50+ classes/professions, that will keep building the world from some jungle or desert to, well it will depend from the players themselves into what they want to build it with the tools and technology the developers are giving them. So, at the end you have one MMORPG with different servers, where every server is a different world to explore with different technologies, different architectures, some torn through wars and diseases, others peaceful and highly advanced.
There is a reason why this genre's been dying, and that's because very few could recognize and acknowledge the potential a sandbox MMORPG has over a themepark. As you can see it takes far less resources to create it and through time it can also become far greater than a themepark can. It's also far less casual, because players can play the game consistently at any time through the day, week and year, not just wait on certain times through the week to get that raid going.
The end game features will only grow through time as well as player driven content will ensure players have endless things to do consistently. It won't be anymore a race to get the best gear, but a journey where the player won't feel he is behind or at disadvantage because he will find himself surrounded by many players ahead of him as well as behind him in whatever he is doing and experiencing. In a sandbox MMORPG with player driven quests it can be enjoyable to just be a peasant or a soldier to some commander, where you can slowly climb the ranks, gain his trust or w/e. Just like in real life it will be up to the player himself to know what he wants to do and experience in the game and how much he wants it to sacrifice whatever else he likes doing.