Ok, thanks for that first rounds, but I think I need to precise a bit more this:

I see that the "imperfect" quality of the system or the fact that is 3d may affect the outcome, but let's assume for a moment that all conditions are "optimal", perfectly elastic ball, no absortion of energy by the wall, no deformation, no 3d tricks (we can always "frame" the bouncing along a single 2d plane) , etc, etc.

so in this ideal system, why should the ball necessarily bounce off the wall at the same angle of impact? then I would be happy to discuss any departure from the ideal system ( I guess I am aiming to thermodynamical/energetic considerations, but I don't have a good answer )