The war aims of all the major powers evolved with the conflict, and tended to become more ambitious and more entrenched as the slaughter escalated and the price of victory become more clear. Germany was no exception to this trend, so there is no single answer to your question. Rather, war aims were a cause of conflict between a predominantly civilian group of moderates, and a predominantly military group of annexationists, with the latter becoming more dominant and more ambitious as the war dragged on.
The earliest German war aims were set out in what is known as the "September Programme" of 1914. This was a document that was apparently compiled, at least in part, by the Chancellor, Bethmann Hollweg, from claims submitted by various Pan-German agrarian and business groups, and drafted at a time when it seemed likely that a relatively easy victory would be won. The September Programme called for
- The destruction of France as a great power, via the cession of Belfort, the western Vosges, the major heavy industrial region of the Briey-Longwy basin – an area it was envisioned would become "the Reich's mineral chamber" – and the demilitarisation of the whole Channel coastline from Dunkirk to Boulogne. These were all territories Germany had coveted, but felt diplomatically unable to seize, at the time of the Franco-Prussian War (1870-1)
- France would be partially disarmed, and placed under an economic burden so heavy it would be unable to rearm for at least 15 years. It would pay Germany an indemnity of 10 billion marks – sufficient to discharge the whole of Germany's national debt
- France would sign a long term commercial treaty that made it economically dependent on Germany, and would agree to cease trading with Britain
- Both Belgium and Luxembourg were to disappear as independent states. Belgium would lose Liège, Verviers and Antwerp to Germany, and the rump of the country would become a vassal state that existed under German military occupation, while Luxembourg would be incorporated into the German empire as a semi-independent state on a par with Bavaria
- Germany would set up a customs union designed for its own economic benefit that encompassed the whole of western Europe and this union would eventually also incorporate the Scandinavian countries and Poland
- French and Belgian colonies in Africa would be annexed in order to create a contiguous central African German colony
After the Battles of the Somme and Verdun in 1916, which cost Germany enormous quantities of men and treasure, German war aims expanded overall, and the army came to have a substantially greater stake in how they were determined. Much of the new focus was on expansion in, and the ethnic cleansing of, eastern Europe, but war aims did change in the west as well in 1917-18.
- Germany became willing to offer a guarantee of the continued existence of a neutral Belgium, largely in an attempt to deter an American declaration of war. In the event of a German victory, this would have involved support for the development of a dual Belgian monarchy ruling over two separate states, one Flemish and the other Walloon, each with its own parliament. The Flemish state, it was hoped, would be diplomatically and economically dependent on Germany
- As part of this policy, the new Flemish state would be encouraged to lease the Flanders coast to Germany
- Belgian transport systems would be closely linked to Germany's and Belgium would be placed inside a German tariff barrier
- Germany would gain access to the major Channel port of Antwerp via lease or the creation of a German-dominated port company
- The planned German economic treaty with France was elaborated on. It would guarantee Germany favoured nation status and enable the Empire to "lay hands on" French Thomas meal and slag, silk, vegetable oils, wool, bauxite, phosphates, chromium and nickel. France would be economically enfeebled to the point where it would lose its industrial capacity and become a mere supplier of raw materials to Germany
Sources
Volker Berghahn, Imperial Germany 1871-1918: Economy, Society, Culture and Politics (2005)
Hans W. Gatzke, Germany's Drive to the West: A Study of Germany's Western War Aims During the First World War (2019)
Fritz Fischer, Germany's Aims in the First World War (1967)
John Horne (ed), A Companion to World War I (2012)