So the first of the three MMO types that I wanted to discuss was the leveling based MMO. Again, the reason I think this splinter MMO is necessary is because in many MMO's, WoW specifically, the leveling game is already a game unto itself. You can solo all the way to level cap without learning any of the skills needed for raiding or PvP. There are no tutorials for that, nothing to prepare you to tank, heal, or manage aggro (an argument could be used for being taught the art of crown control). Therefore, we should encourage developers to take that portion of the experience and flesh it out into an entire game instead of pulling a bait and switch with everyone who makes it to the level cap OR, conversely, forcing people who just want to raid/pvp to drudge through content they don't want and that doesn't effect endgame in any way.
Now, when you get rid of the carrot and stick of endgame raiding, it gives you the freedom to expand the game horizontally instead of just adding vertically, or, to be more specific, giving the players more to do as they level instead of making them work through the world's longest tutorial to get to the good stuff.
Since the players would have more to do (I'll get to what they could do in a minute) you could slow leveling down. Waaaayy down. I imagine it as the hardest of the hard core reach a soft level cap (where you are still getting xp but such a small amount that it isn't really the point anymore) by the time the next expansion is released. In fact, you may not actually require leveling at all. Maybe you could do it all with gear and powering up abilities, so for example, instead of an area being inaccessible to you because you haven't reached level 15, maybe the monsters there are just too tough for you to kill on your own without better spells or better protection, the latter of which could come in the form of better gear or a larger group.
I imagine this game being largely exploration based so the world would have to be truly massive as in it might take you 24 hours of continuous play to walk all the way across one of several continents. That isn't to say that there couldn't be faster modes of transportation, but more to give the feeling of a living world that's much bigger than the player. Imagine going for a ride across the landscape in a game without having a necessarily hard line between zones where you might run into an easily farmable mob or you could run into a bandit's hideout that would clean you out. You could give the player an in-game map that they could annotate themselves to mark hot spots or, maybe an area where they were likely to find a crafting item they regularly use or any one of a hundred things. The point is, you don't spell it out with giant floating arrows. You leave it up to the player to explore the world around them and focus development energy on making that world as vibrant and engaging as possible.
Of course, there will still be questing. I see that as a major component of game play. This is another area that could be expanded. Blizzard has done wonders with creating involving quest chains through the use of phasing and mechanics that go beyond "kill ten foozles" and "go grab that maguffin" but in a game where that is a major source of game play, they could push it further. MMO's have done this in the past (EQ2, I think had an aspect of this) but why not let the player make decisions during the same quest (besides whether or not to do the quest) where they can ingratiate themselves to one of two or three NPC factions fighting for control of a given area? The choices you make could set you on a path, but all the while giving you chances to change loyalties as more of the plot of the quest is revealed. This adds a little bit of Role-Playing where you can decide what aspect of a conflict your toon would relate too/support. With this, you could also have quest hubs that change hands between these factions and it could work as a form of pvp in that players in some areas might support different factions and could end up on opposite sides of a conflict.
Grouping will always be an important part of MMO's. Otherwise, you could just be playing a single player game with an online leader board. In this version, grouping would show up in the form of a more traditional adventure party: a group of mixed characters, not necessarily a WoW standard 1tank3dps1heal set up, but more bringing a diverse group of classes so that you can deal with a greater variety of situations. There would be no penalty for grouping other than the obvious one: everything you get is split between all party members, save gear. This way, there would be an advantage to both a smaller group (fewer ways to split the expedition's profit) and a larger group (able to reach tougher places that would yield better rewards.
Another good lateral way to expand the experience's legs is a more robust crafting system. Imagine a game where the best gear could only be obtained by bringing rare materials to a local artisan? It gives broader purpose to the exploration and also gives people the option to trade on their crafting skill. In WoW, all it takes is a button press and materials to create any item in your book of recipes, but imagine if there was a lot more to it. A crafting mini game is one way to go, though that might over complicate things. I think a better approach would be to not have recipes, per say, but to have a far greater number of materials and have multiple stages to crafting a single item. Let's say you wanted a better sword. You find some ore, some leather, some gems and some magic materials. You take the ore to the forge and have a master forge smith smelt them into bars for you (the higher his skill, the more and better quality bars you can get). Now you take those bricks to a blacksmith who can hammer those bars into the shape of the sword you wanted (a big two-hander, of course). The better the smith, the higher the quality of the sword and the more jewels and enchants it can hold. Now you take that to a leather worker who can process the skins you brought, and fashion the grip (the better the grip, the higher your accuracy with the sword). Lastly, you visit an enchanter and a jewel crafter (and a cook for a sandwich while you wait) and they attach and imbue your new sword with all the powerful gems and magical powders/essences you gained from the auction house or through your adventures. Last step: you look at the final stats of the weapon, bask in its glory for a moment and then have the option to name it. Now, it's a much more involved process, but it gave a dozen people work to do and, I would imagine, the player is much more attached to a sword that is the product of all his adventures, the work of several artisans and that he named himself (or herself, I guess). Better than a giant Hydra dropping a pair of boots. On top of all that, you have the industries and quests that could arrive from the collection of rare materials and the transporting of those materials from one city to another (the auction houses would not be linked, so prices of some items would be lower in areas where that type of item is plentiful).
Add to that player housing, pets, mounts to collect, guild halls to erect . . . there could be a ton of content and no endgame necessary. As with many MMO's, playing the game becomes the point of the game. You can build your own story.
Next up, the Raid-Central Based MMO!