So far, I've outlined a true sandbox MMOG with 24/7 character advancement for everyone. Some of the side benefits of this game paradigm is that you can't have characters getting to level 50 in 4 days and completely skewing your character advancement forecast. Everyone proceeds with equal advancement, with a catch-up system for those that begin later (as well as being able to begin fresh and equal on new servers that come up). This system also well serves people with busy schedules and who wish to play other games as well.
However, since players can have their avatar earing money as part of their continuous directive, one wonders about the economy and how it can be sustained. This is where "sandbox MMOG" takes on a whole new level of meaning.
Envision a system where various crafting modules can be purchased such as a armor smithing, weapon forging, tailoring, spellcrafting, enchanting, alchemy, pet trainer, physical training (providing new emotes and motions in and out of combat), magical tattooing, music & musical instruments, construction, interior decorating, chef, etc. In other words, modules do not increase game-size mostly along a linear path like most expansions (more levels), but can broaden the game tangentially. Although expansions can incrase the linear size of the game, let's remember that the speed at which anyone can progress is set. Nobody can "speed through the game" faster than anyone else, because everyone advances 24/7. The game will have both expansions and modules.
These modules will coincide with in-game advancment in relevant areas. In order to be able to buy (purchase and dowload) the armor-smithing expansion, you must have opened that area up in your advancement in the game and by performing a few tasks or quests live. The module consists of a sequentialized set of construction tools the player gains access through by advancing his character in that field of instruction. As the player unlocks more and more of the levels of the module, they open up more and more options, abilities, style, power, materials, etc. in the crafting/job/service module.
These modules will be mini-construction set tools where the player can design their own product and have great creative control over the output. They can then sell product or train others in the game, thus creating a vast market based on the creative talents and skills of the player, and the amount of advancement time they've spent in that particular avenue of their character's progress.
Furthermore, I suggest that player accounts be set up with a PayPal-like real cash system, where products or services they offer can have prices set for in-game gold or real cash values. Instead of the game producer setting up a shop where players can purchase some game items, I suggest that the game producers simply take a percentage of each player-to-player sale (nothing that each item for sale must have a minimum value for this to be a viable system; items below that value will simply have to be transacted in game money).
It might be that a game system like this would be very profitable even if it were free to download & play, with the players paying real money for crafting/job modules and the game company making a percentage of player-to-player sales, but I think it might be better to initially charge for the game to generate some immediate capital, but not have any subscription fees.
[Edit: I think that on further reflection, i think that since one is getting 24/7 advancement, I think it warants a modest fee. Also, the real cash aspect of such game modules may or may not be a good idea, but the overall structure of the player being able to actually design, trademark and upload content for the game is, IMO, a sound one if feasible.]
Next: A deeper look at the crafting/job system.