So on the 3rd of March of 1918 the Treaty of Brest-Litovsk was signed formally ending war between Germany and Russia. The Treaty seen huge swaths of land handed over to Germany and Austria-Hungary. The figure I've read was something like 780,000km². What were Germany's plans for all of this gained territory? Wikipedia states that the land area in control of the German Empire in the years up to WW1 was 540,857km². They gained territory larger than their own empire. I feel as if this may not have been sustainable. What were there plans with this territory?
If you look at the German Septemberprogramm, issued at the beginning of the war, they planned to create an economic association known as "Mitteleuropa." This would essentially be an economic union of states in Central and Eastern Europe under heavy German dominance. With the land gained from Russia they would have likely carved out buffer states that would be subject to German influence.
Source: http://www.wwnorton.com/college/history/ralph/workbook/ralprs34.htm This is the translated text of the Septemberprogramm. Note that Poland did not exist in 1914, implying their intention to carve it out of Russian land.