Hmmm I think the starting point would be to define what the devs want to be the essence of the experience in the MMO, the core I was mentioning in my previous post, and from there, work the context in which this experience will be presented, and what variables this context offers.

For example if the core of the game is the character progression, things as experience rates or gear gains (everything that makes the character advance in one way or another), should be static, but in this example, the amount of solo or group content could be variable, meaning you could have servers with harder mobs that encourage a FFXI grouping style gameplay, and others where mobs are easier thus promoting solo play.

If the core of the experience is the story, the the character progression is a variable and could be changed.

Is gear gathering the core? pvp? exploring?

If the intention is a mix of all, the we have a problem because the whole game is the core, so no variables would be left to change...