I see it working this way:
GM1 of W1 declares that his world has three portals to W2. GM2 of W2 then decides where those portals emerge, whether they are two-way or one-way, and he can have several of them have exits in the same place. For example, GM2 answers that his world has one two-way portal with W1. Therefore, when a character uses any of the portals from W1 to W2, he ends up in the same place in W2. When a character crosses back, he ends up in one of the three locations in W1 at GM1's discretion.
If we do this without a hub, could open a google doc to represent the planets and their connections visually, with the portals orbiting the 'planet' a game is placed upon.
A possible result of your example represented visually:
Colored are to represent that while those cross a line somewhere, they aren't connected with that line.
Yes, that is what I meant. It seems to me that the more control a GM retains over their own game, the easier this will be to manage. This way the only thing the GM has no control of is
whether and when players are going to enter his world.
Where will they enter and leave remains for the GM to decide.
On the subject of psionics: I think every world has its own set of physical/metaphysical principles. Most worlds are going to have basic physics similar to those observed in real life, plus some others regarding magic/psionics/divine intervention/what have you. If an ability that crosses worlds ends up lacking something to support it, it gets nerfed. However, a character who is a denizen of a certain world is themselves a bit of that world. Therefore, if a mage gets transported into a world with no mana, they can still use the mana within them. A psion, however, is more complicated. I see two possible solutions.
1. Objects in the psion's home world have some sort of inherent property that allows the psion to control them with their mind. That means objects in another world won't have that property/will have it to a much lesser degree. So a psion will have to exert much more energy to affect otherworldly objects (including those brought to their own world - interesting possibilities) or be straight out unable to affect them (this way a psionic telekinesis user will only be able to move objects from his home world around when in another world. Also interesting).
2. A psion affects stuff using some sort of mental energy thingy. Then it's the same as mana, and their abilities get treated the same as magic.