Some brief notes on the concept of plane shifting. Planes could be categorized as following:
1) Temperature (hot planes vs freezing planes) forcing shifters to (un) dress for the weather 2) Fundamentals (metal-less planes, explosion-less planes, electricity-less planes, magic-less planes, etc) 3) Gravitational (low vs high gravity) shifters would be the strongest in low gravity planes, where as high gravity planes would be dangerous. 4) Technological (low vs high technology) technological capability of sentient natives, if any. 5) Darwinism (low vs high energy) Utopian planes vs Dystopian planes as to the aggressiveness of the ecosystem, could make high-oxygen planes more aggressive.
This is just a brief list. Shifting between planes could be done by entering a number, which is randomized by seeding a random number generator with a value unique to the player. As there would be roughly 1 billion planes, and shifting and exploration could take half an hour per plane, this would make most planes only accessible to a specific player.
Alternatively you could display this as a grid that has 7 intensity levels, with a total of 7 scales, which would allow for 800 000 planes. The player could be given plane shifting commands that will raise and lower values, but it's important to raise and lower multiple values at the same time. So if a player uses 'shift blue' it could lower temperature, lower gravity, and raise technology. In order to reach a specific plane the player would have to use a complex series of shifts, shifting through several planes in order to reach a specific plane. If you want to make shifting unique to each player you could shuffle the scales along the x-axis, which would give few players identical shifting combinations.
Some degree of physics handling and world generation would be required. Worlds might be semi-persistent, allowing players to build fortresses and treat a plane as their private property, possibly converting the natives to believe him a God and attack false Gods, and lead large armies of followers through various planes to engage in inter-planar warfare, as described in the Amber books by Zelazny.