Germany's financial situation (just as Britain's) was in shambles and any war is going to be followed by a post-war depression at least for 2 years or so as the economy adjusts to peacetime. Additionally, Germany is going to be busy propping up its allies and puppets in the East, so by 1918 colonies might not be a major immediate objective.
Back to the initial question at hand, I think the Pacific colonies are lost as Japan will not return the Pacific Islands, and it is of little value for Germany to pick a fight over them. The same goes for Papua, Samoa and Nauru as the Australian and New Zealand governments were more independent by 1918 and would refuse to return the territory. Again, for Germany to launch an invasion with these countries over small islands is probably not worth the trouble.
The easiest colonies to obtain would be Kamerun and Togo. In East Africa, von Lettow-Vorbeck was still fighting with hit and run campaigns in Northern Mozambique and later Northern Rhodesia. Returning German East Africa to Germany in its entirety might be possible in exchange for some other concession from the Germans regarding the Channel Ports. Obtaining Mozambique North of the Zambezi from Portugal is also a real possibility. Portugal itself is weak and its finances were in shambles due to the war so they might have to cede Mozambique south of the Zambezi to Rhodesia and the Union of South Africa.
In South Africa, the Smuts government also was far more independent and told the British government that Southwest Africa would not be returned to the Germans. Afrikaners made up around 25% of the European population prior to WWI and again Germany might be willing to cede this in exchange for the Belgian Congo and even Northern Angola. A possible partition of the Belgian Congo is possible as the planned Katanga Railway linking the Rhodesias to Atlantic would most certainly place Katanga within British control giving the British Angola south of Luanda so they could control the port of Lobito and the Benguela railway. The islands of São Tomé and Príncipe might have also been of interest to the Germans, this again depends on the British being willing to negotiate it away.
For the French to gain some concessions in Europe they may cede most of French Equatorial Africa, to join it with Kamerun and the majority of the Belgian Congo to form a German Mittelafrika. Dahomey, Madagascar and even French Morocco are other territories that the Germans would possibly be interested in. Though Germany would have lost its Pacific and Asian colonies, the establishment of a large colony in central Africa would have been seen as a victory at home.
First I think the financial situation for Germany is not that bad, the bulk of her war debt is inflated currency and internally held, paying it down simply plows money back into the economy so long as one can contain further inflation, prevent deflation and curb capital flight. It isn't pretty but it is not ruinous. Germany needs to fire up industry and get back to exporting as soon as possible, that means it needs foreign currency, gold or credit, soon rather than later. I think you see austerity like post-WW2 Britain as the industrialists are prompted to go earn hard cash. Worst case you see the same barter style trade that pre-WW2 Germany evolved to work around its weak currency and lack of gold. So more trade with China, South America, the other places where hard goods can be traded for raw materials. That is a direct threat to the London financial empire and gold system, so Britain needs to engage Germany before she builds too much independent trade. And if the USA is warm to it, Germany likely gets loans and raw materials from her, circumventing the British and kicking off a profitable relationship outside British control. Thus for me the colonies are far less vital.
The USA will not prefer Japan gaining the Pacific islands, we might have Germany sell them to the USA to raise cash, that would be a twist. OTL the British backed Japan getting them to curb American growth in the Pacific, the USA conceded in part because as Japanese territory they could freely be used in war where if still German they are potentially off-limits neutral ground. I toy with the USA getting leased bases in the Islands to draw Germany and America together opposed to Japan.
I think any division of Portuguese or Spanish colonies is in the future and not likely to be put forward in the armistice at war's end. I agree that if more needs given it comes out of France's possessions. And I like Germany demanding stuff that Britain must press hard on SA or Australia to cough up, that is good diplomacy, further breaking the unity of the Empire, even if it fails the Germans have made Britain lose something pressuring them. I think push come to shove the British can force the Dominions to yield but it will accelerate the demise of British dominance in the Empire, it should short circuit any Imperial Preference working, and if we give the Germans that much skill, they have set up a transition to becoming the alternative partner. Australia will see the USA first and next Germany as good friends in opposing Japan, SA will have a German counter-weight in the rise of Afrikaner political power, the fate of Rhodesia and how decolonialization proceeds. Like Canada, SA might quickly become more about trade with Germany than the UK, and like Canada it might finds its interests at odds with London very quickly.
To guide a far better style of administration I rely on Wilhelm Solf getting a bigger voice. Who in the German Foreign Office has any vision at all? We need a very crafty player to get us so much out of the colonial fight.