I have considered the following concept on and off for a number of years. It can be a little confusing to describe to begin with, but hopefully I can pull it off adequately.

The Concept.
Imagine that there are an infinite number of parallel worlds surrounding us. We move through them blindly, with every action changing our balance and alignment with the world around us.
Now, imagine that we are not so blind, that we can see the adjacent worlds as ghosts. We can interact with and manipulate those ghosts, but doing so will pull us into that world and away from our own.

Making it work.
You start with ten identical zones. This is not a fixed number, but just for demonstration purposes. You start off in zone 5. You can see things in zones 4 and 6 as solid looking, but translucent objects. You can also see some items in zones 3 and 7. but they would be very 'ghostly', and mostly see through. You have a sword, manufactured in zone 5. The sword does 100% damage to anything else that is within zone 5, but only 66% damage to anything you try to hit in zones 4 or 6, and only 33% damage to those objects in zones 3 and 7.

Every time you interact with something from a different zone, you get pulled more and more into alignment with it. This would apply to all objects as well.

You see a mace on the ground, it is slightly translucent, it is an object from zone 4, so you pick it up. Just by performing this action you get pulled into alignment with zone 4 by a small amount, and the mace gets pulled into alignment with zone 5 a little. You go around, and kill some critters from zone 5 with the mace. It is only doing 66% damage to start with, but the more you use it, the more damage it will do, as it is becoming more aligned with your zone. Your actions with the mace will try to pull you into alignment with it, but as a counter you are hitting objects in your current zone, so you will stay 'anchored'. If this continues, eventually the mace will end up as a part of your zone 5.

Instead you decide to hit critters in zone 4. The mace will do 100% damage to those critters, although it will lose some alignment the more you use it. However, because you are continually interacting with items in zone 4, you will get sucked into that zone. At this point zone 5 will seem slightly translucent to you, and you will be able to see zone 2 items as ghostly objects.

Being 'attuned' to a zone would be a similar experience to how skills would work. Everything on you would slowly lose it's adherence to it's original host zone unless used on objects within that zone. Keeping a zone 4 item attuned to zone 4, while the player is in zone 5, would require you continually use it on zone 4 objects, as well as the player constantly interacting with zone 5 to stay attuned there.

Death and Taxes
The game would be full loot by design. Due the the entire 'parallel worlds' concept, we also have an easy way to deal with and explain away death. It is one of my pet issues that most games just resurrect you with no real explanation as to how. Just because you are killed in a fight, it doesn't prevent a 'parallel' you from coming too in the middle of a battlefield, suffering from a flesh wound that miraculously failed to kill you. Thus you would be transported to a 'safe' parallel location, only carrying items that are bound permanently to your soul, with everything else having been dropped where you died elsewhere, and looted by the pesky attackers. This would also help answer answer the whole issue with regards to why players would want to travel. Maybe you would just want to get back 'home' and defend your slice of the multi-verse, or to enact revenge. This is an important part of the core concept of the game design.

Skills and Levels
What levels? You learn skills, either from other players, game objects or NPC's. How you utilize and combine these is up to you. You can be a magic wielding, plate armored, psychopathic killer if you really want to. There would be skill levels of a sort, but not using a skill for a while will also cause you to lose your edge. You may have been a master swordsman last year, but this year you are out of practice as you have been using that really cool Quarterstaff you attained. Skill levels need to be actively maintained. The same for magic.

Server Model
This is the difficult part, especially as a lot of it will come down to load balance and synchronization issues. Ideally each zone would run on it's own server, but this is not a realistic possibility, and synchronization of potentially hundreds of parallel servers could bring things to a halt quite swiftly. In practice you would probably end up with a single server/cluster processing a group of between 3 - 10 zones. If things become busy, it can offload low population zones to other servers or cluster as needed, with priority being placed on keeping the adjacent zones in sync.

Hopefully this gets the basic idea across. When you start to bring player vs player interaction into things, or multi-zone groups to defeat bosses, thing become far more mind bending. Imagine trying to fight a PvP battle where you can only see half of the players...

Your thoughts would be nice, and also feel free to ask specific or general questions as to how so and so would or might work.

Originally posted by Drafell

 Imagine trying to fight a PvP battle where you can only see half of the players...

Thoughts would be nice...

That would be any given seige in WAR. :)

Yeah, but that's not really intended in WAR :-P

No doubt, however the player reaction would be much the same, intended or not, no?

It sounds very awesome to me,some outside the ole' traditional mmo thinking here.This could be very immersive and engaging a real refresh from the same ole' drudge that exists in most mmo clones. I like the idea's you have presented and will like to comment further as I have time to think about this idea Nicely Done:)

zzzzzzzzzzzzz
zzzzzzzzzzzzz
zzzzzzzzzzzzz
zzzzzzzzzzzzz
zzzzzzzzzzzzz
zzzzzzzzzzzzz

I get the idea but don't know why players would want to enter into new worlds.  What would the motive be?

Originally posted by CactusmanX

I get the idea but don't know why players would want to enter into new worlds.  What would the motive be?

What he said.

Might work as a secondary feature, with an astral plane or what not, but as a pure concept it's a bit confusing.

Essentially you have parallel worlds. You might die on one, only to wake up and find you were not killed in another, it was only a flesh wound. Soulbound items would then have another, entirely different, role. They are bound to your soul, not to your body.

Added some expanded text and information with regards to levels and skills to the OP.

I forget the name, but there is a chinese/japenese/asian RPG that came out just a bit ago that has something like that. Its always online, but you never actually play with other people, they just lobby into your game and can leave you little notes helping you and put down some signs on how to beat a boss or certain part. Really cool idea imo.

Being 'attuned' to a zone would be a similar experience to how skills would work. Everything on you would slowly lose it's adherence to it's original host zone unless used on objects within that zone. Keeping a zone 4 item attuned to zone 4, while the player is in zone 5, would require you continually use it on zone 4 objects.