What were Germany's war aims in the western front?
Early war: Knock out france decisivly before Russia can fully mobilize
Mid war: Hold onto current gains, make some rather limited offensives
Late war: Acheive a decisive breakthrough before Germany implodes
As for any gains, IIRC there was never one cohesive, accepted collection of German war aims. Depends on who you ask, and when. Germany sort of rushed into a western war without thinking through what it actually wanted to gain from the experience. Probably, annexes/makes a protectorate of belgium, some gains in northeast france, a few colonies, and probably a major war indemnity. Britain is harder to speculate on, because this depends on how (and if) britain is involved in the war.