Modding the lighting and specular as in idtech

[Abion47]

Why there isn't access of the C++ deeper capabilities?

Because it isn't C++. AngelScript is a sandboxed scripting language, meaning that it is only as powerful as the game allows it to be. While FG has made HPL3 pretty damn powerful, it's still just a scripting language at the end of the day, and it cannot be as powerful or as flexible as a compiled language, particularly one like C++.

Only specular bump map and lighting rendering.

What about particle system ?

I'm not sure what you mean by lighting rendering. HPL3 uses light sources and supports specular maps. It also has a fairly hefty support for particle systems, even going so far as to have a dedicated editor for designing them.