OK, so how does magic actually work in the planes?
AD&D rules are clear on this in the original books, okay, but how does it work in 3.5? I don't think some things are entirely clear or explained in sufficient detail.
For instance, how do summoning spells work when you're in the planes? From what I understand; when you're on Prime Material, you whisk creatures away from other planes by means of summoning crystals.
But where do summoned creatures come from, exactly? OK, 'planar ally' clearly summons them from the Outer Planes, and the elementals are called forth from the Inner Planes.
But let's say you're in Gehenna, and you're a druid. Can you summon animals as per normal? Where do they come from?
And if you're a wizard? How much harder is it to summon a guardinal to the Abyss than a gehreleth?
And if you're a cleric, and your deity's HQ is in Arcadia, how does this affect your spellcasting abilities while in Pandemonium?
...
You catch my drift. These things are explained in the 2nd edition, but remain unclear in 3rd (and 3.5, which I'm currently running). I like the idea of having to keep check where you are in the planes and how that relates to your spellcasting, but copy-pasting from 2E just wouldn't work.
And another thing. What about weapon enchantments? There's that thing where enchantment weakens as you move from a major group of planes to another (i.e. from the outer planes to the astral, and then to the prime material, etc.) Do you think I might implement that in 3.5, and how?
Well, the Summon Monster spells are stated in MotP not to function without the Astral. Summon Nature's Ally spells still work though. Of course, I would severely limit my mixing of Planescape and 3x magic and continuity rules since, if you used the Planescape cosmology with 3.5 spell rules, a LOT of spells would become unavailable on the Inner Planes-- including practically the entire Teleportation subtype of spells. Whereas Force subtype spells would be unavailable on the Outer Planes since they tap into the Ethereal. I'm not kidding.
Personally, I use the Planescape/2E rules for coterminous planes/cosmology and summoning/magic, in addition to the 3x "psionics are the same as magic" rule since I don't exactly feel like giving everyone separate Spell Resistance and Power Resistance stats, nor do I feel like conjuring up 20 more spells and powers to affect the other. In fact, I treat all non-item supernatural abilites as being affected in the exact same way by the planes. This means that Desert Wind school maneuvers with the Fire subtype aren't going to work on Elemental Water or Paraelemental Ice without a spell key or similar such object.