There's really only a couple unclear points in the original Phantasmal Forces spell:
- Vision only, or multiple senses?
- One image, or multiple?
- Is the "disappears when touched" reciprocal?
And it doesn't really matter what your answers to those questions are, as long as you're consistent. We're talking about a made-up thing, remember, so you can define what it means in your game world and that's it, problem solved. I think people are afraid of players challenging their interpretation. That shouldn't be a big deal, since the proper response is to openly mock the player for challenging your interpretation of something you made up. One of you is being far too serious, and the great thing is: you get to decide.
Some people will probably raise the objections that disbelief and damage is tricky, too, but I think that's because they're interpreting these as mechanics, when they don't have to be. Sure, the JG Ref Sheets include a tiny table for "roll to disbelieve", and The Fantasy Trip's approach was to make this a formal IQ roll. But there's actually no reason to make it a roll unless you want to. The point of the comment about disbelief was to make it clear that illusions aren't real, and that a victim of an illusion might believe the pack they dropped is on fire and being destroyed, but since the pack itself doesn't have a brain, it can't be fooled by an illusion and therefore cannot really be damaged.
The imaginary cup I mentioned in the last post was given as an actual example of a "hard" problem when handling illusions. I still can't quite cope with the ridiculousness of that suggestion. It's not a real cup. Of course it can't hold water. Nor can you create an illusion of a staircase so that you can reach the top of a cliff easily. You can make someone else think they are climbing stairs to reach the top of a cliff, but in reality they're just kind of thrashing around, flailing their arms and going nowhere.
Now, you can invent a new spell for your campaign that creates illusions so real, they can interact with physical reality and not just your brain; but again, that's something you're making up, and you are responsible for your creation. Have some conviction in your beliefs, if that's the way you want illusions to work. Stop complaining about how hard it is.