Please, hear me out before you lynch me. From what (admittedly little) I know of the Planescape setting, the Prime Material Plane contains all of the various universes of "standard" D&D, with each universe sealed up in its own Crystal Sphere and only reachable by: A; powerful magic, B; using extraplanar portals or C; Spelljamming. Following this understanding, it is thus possible for a Planescape game to include Primes from Forgotten Realms, Dragonlance, Greyhawk and homebrew campaigns, correct?
The point I'm getting to is this; it's quite possible for a Planescape party to include PCs from different Prime settings, but what about including Primes from more "modernised" Crystal Spheres? Primes from the worlds covered by D20 Modern and its various expansions (D20 Future, D20 Cyberscape, D20 Apocalypse etc)? Would you allow or disallow such player characters? Especially if they brought along a toy or two from their Sphere- such as an energy weapon or even something like a pistol or Uzi.
In essence, my basic question is this; is it possible to "integrate" D20 Modern-based Primes in a Planescape game, and if so how would you do it? There's an idea about the integration of Mechamorphosis into a Planescape campaign rattling aound my empty head, but I'm not dumb enough to think about that!
Basically, the Prime and the planes are all connected, so the greater planar society incorporates pretty much all technology discovered on the various prime worlds. Therefore, the only prime worlds that would have more advanced technology would have to be almost entirely cut off from the rest of the Multiverse. Even then, you should probably limit it to a late-renaissance/pre-industrial revolution level of technology for a few reasons. One, it would be unbalancing for one player to have an uzi and another player to be stuck with a pointed stick. Two, Mechanus is widely regarded (with good justification, as it imbodies the scientific method) to be the pinacle of technological innovation, and shouldn't be supplanted. Three, if players are given access to effective industrial wonders like electricity and repeating rifles, magic swords seem a lot less interesting.
But don't fear, there is a better way! If you want to kneecap people on the planes, you need look no further than the "Urban Planescape" section of these forums. Basically, in UPS, mutiverse has moved forward a few hundred years, and now everyone really does have tanks, guns, and Inter Planar Ballistic Missiles.