but truly stat upgrading is completely up to you. That's what I've always personally liked is to be able to literally model my character after exactly what my play style is.
I feel the same about roles. And that's exactly the reason I'm looking for a way to let the player advance as much as he wants. But if he takes his role seriously by obtaining all the perks and becoming a true specialist, I want the game to open new possibilities to improve his character.
make them [Achievements] so that they are personal trophies in the player house maybe
That's an excellent idea. Indeed I already had another idea that expanded the role improvement path, but I needed a way to attach it, and this idea of substantial trophies in the game allows me to do exactly that. I'll expose the concept soon.
no matter how many times I play the game something different is likely to occur.
That would be great, but how do you accomplish that? I gave you some reasons (in your own thread) of why this is difficult to achieve, but it's in an interesting topic that should be discussed more. As I told you, in my case I want to allow the player to avoid missions and to concentrate in developing his own role in the game's world.