I'm just a newbee in MTA, now I'm learning the wiki.
I've found the function SetElementDimension with description for the argument Dimension: "You can also use -1 to make the element visible in all dimensions (only valid to objects)."
So, I have a question, why only for the objects? Why not for peds(players) and vehicles?
So, it would be cool to have a dimension that everyone will see but won't see each other, like if two players would be in one apartments, but in two different dimensions(eg. 1 and 2), so they wouldn't see each other, like they are in two different apartments, but they would be able to see everything that happens in the street(-1 dimension), cars, walking peds and so on. At least, it seems to be cool :)
If I'm not mistaken, the apartments in GTA Online works this way.