After reading this, I can think of plenty of things that could be blended into a persistant world, be at its core one thing, yet blend a few things from other sources....For Instance..
You want a RTS...You want 2 or more sides, preferably more than 2 Id say.
You have Savage and a few other games of the same type where you have RTS blended with RPG
Now Stick in the MMOG and Muds, and heres what I think you could get...
(for lack of name, ill use style of game as its name) Start with a persistant Savage world. In each world, your character, when created, gets a patch of land somewhere on the perimeter of wherever. You get standard buildings that are your own. You are ruler of your own patch of land, have a Major Domo to see over your 1 acre while your away (or you just start out as a mercenary, then build up enough resources to purchase your 1 acre) Now for sake of Growth, this world is ever growing, the world itself Expands to fit its occupants. As you gain in power, your landsize grows, you buy/build more buildings, gain more 'abilities/resources' to draw upon. Bring in the Mechwarrior Mercenaries/Freelancer type of game, where you can become a gun for hire against the other side, or become the boss who hires (probly you start out a merc, then you hire as you get more powerful etc.) You buy weapons, you outfit a company of 'hired goons (live or monsters) and run your scenerio into the other sides areas.
Now theres a few ways it can split from here, your lands generate income/resources, and also have debts to pay, so theres one limiting factor of certain things. You can go on quests for items/resources/heros or hire someone to do management, gain benefits/disadvantages to fit within your own kingdom. You buy Defenses, bodyguards, town guards, police, boarder patrols, outposts, etc as you get bigger and need it. Your town in essence will become a 'zone' like in any MMOG for the other side to come raid and pillage.
There are a few problems with this of course, but they could be worked out, lets just think of the framework.
Now, magically, your area may switch places as you become more powerful/large area or some other system, perhaps they all become islands that move, who knows.
Now for the RTS part when people are away. Your town is already a defensive keep, with the ability to hire people for missions or maintain a standing army. the other aspect is buying buildings that spawn units that rush at the other sides boarder (collective boarder, however you do a PvP boarder) which has defenses built up with money from the side as a whole, from conquests from the one side, from beating back the enemy and having monsters loot the treasure from the attackers. I can envision a maelstrom constantly in play in the center if sides are even in strength, and one side getting stronger due to players being crafty or helping out, or just making a better strategy of units, and packs of spawns.
So, you have the center battle, you have ways to get around that battle and infiltrate the other side, and sabatage stuff (either permanently, or temporarily depending on whatever it is they sabatage) the posibilities are endless, people have to pay to repair, or can get other players to help repair, and players could be the repair people in game, charging for services, etc...Bang, in game economy, with shops that build the upgrades, the buildings, the repairs etc...
The limiting factor for making one side have more people than another could be one of the larger one side is, magically its harder to plant a plot of land there because of crowding, but if theres symetry in the magical planes, getting into one side or the other is simple, but if one side has 1000 players, and the other 500, you hit a brick wall of force that wont let you go to the 1000, but will easly allow you entry into the magical world of 500...
I can envision defense systems that limit magical power on items entry into thier domain, I can imagine magical or mechanical ways to subvert those protections, I can imagine alot of stuff in this persistant world...It could also have a 'goes till one side wins', but is geared for a game that lasts for at least a year due to the population limiter stuff etc...or with the ole 'cornered cat kicks 2x arse' play...
So, it is definitely dooable I think in the terms that would make it successfull....
Oh, and I copyright, and trademark, and whatever else my ideas may catch on in a game as a whole, so if you want to work with it, be cool and get me involved *grin*