Is there a 3D particle generator that is 3D in AE 3D space, not just within the plugin itself?
There is a well known particle plugin that for some reason is only 2D in AE space. This makes it difficult to make the particles interact (go in front or behind) with other 3D objects in other layers. A typical trick is to sandwich the object layer with 2 indentical particles layer; with one layer generating the particles in front, and the other layer generating particles behind. What if there are multiple objects in multiple layers, this becomes very tedious.
Is it impossible to make a true 3D particle plugin?
Well, Particular has Obscuration Object options and adjustable visibility ranges, does it not? The rest is of no consequence - if you are looking for full 3D-interaction, learn a 3D program. Like it or not, there is no such thing as a contiguous 3D space in AE and all plug-ins can do is work within their own little universes. It's a limitation of the AE effects API - it simply does not provide any such features, so until that changes - no, not even a genius likje Peder Norrby can change any of that.