The problem with a Brest-Litovsk scenario is that Chiang refused to make any concessions with Japan, and Japan was not able to force him. However, if Japan was able to inflict an even worse defeat on China earlier in the war, then Chiang might be forced to sue for peace.
Here are the major battles that Japan must win to create a situation where Chiang feels he must make peace (or enable a pro-peace faction to oust Chiang).
1) Battle of Taierzhuang March/April 1938. This was a major Chinese victory early in the war that made China feel like it was possible for them to defeat Japan. A Japanese victory here severely erodes Chinese morale, and enables them to hit China harder in the next year.
2) Battle of Wuhan June-October 1938. The battle was a Japanese victory, but came at a very high cost and delayed Japan from pushing deeper into China. A stronger Japanese victory would inflict higher casualties on China, reduce Japanese casualties, and enable them to move up the Yangtze.
3) First Battle of Changsha September/October 1939. The Chinese stopped a major city from falling to Japan for the first time, and prevented the Japanese from linking up their central Chinese territories with their southern conquests.
These battles created the stalemate that Japan found itself in prior to 1941. Victories here would eliminate that stalemate. Such victories were theoretically possible, and so are within plausibility. If we imagine a timeline where Japan won Taierzhuang, quickly seized Wuhan, and then was able to take Changsha that enabled them to link their conquests with southern China, and threaten Chungking, then the entire Chinese military effort might have collapsed.
The actual land ceded to Japan might not be that extensive, but China would become a pupper of Japan, and Japan would have major extraterritorial rights.
Of course, the people of China would hate both the discredited Nationalists, the pupper government, and the Japanese. I'd expect most opposition to flock to the Communists, and the long term prognosis would be guerilla war and eventual civil war. I don't see the puppet Chinese government exercising much authority, and we'd see a return to warlordism. Some of the warlords might remain anti-Japanese, but there'd be little they could do. This would significantly delay China into becoming any sort of world power for a very long time.