First, Poland couldn't exactly "give in" in terms of Danzig, as the city wasn't part of Poland, but a "Free City" nominally under the control of the League of Nations. This in itself caused tension between all three parties (Poland, Germany, Danzig), as no one liked the arraignment, and it didn't really solve any of the issues (which were that Poland wanted secure access to the Baltic Sea; that Germany wanted access to East Prussia; and that the populace of Danzig, some 95% ethnic German, wanted to remain part of Germany). Because of that Poland was not willing to let Germany gain any more access to the area, which aside from wanting to annex Danzig (while not part of Poland, the Poles did have a say in its foreign affairs and would have to consent to any change in its status), included a highway and/or rail line across the so-called "Polish Corridor" that would effectively cut Poland off from its newly-built port in Gdynia (right beside Danzig and built so Poland had total control of a port).

There had been negotiations and talks between Poland and Germany for years over the status of Danzig, and despite several pleas from Germany, Poland was not interested in changing its position. This of course only served to further agitate the population of Danzig, who elected the Nazis to power right after Hitler became Chancellor (I believe they were the first region to do so), and maintained high approval ratings of the Nazis leading up to 1939. Poland feared that if Danzig was fully in German hands, it would totally restrict their access to the seas, a not unrealistic concern; in 1920 during the Polish-Soviet War, dock workers went on strike and refused to unload munitions that were crucial for the Polish war effort, nearly costing them the war (this led to the creation of Gdynia in the mid-1920s, which only further agitated Danzig; it cut off revenue from the city and being only a few kilometres away was very symbolic).

By March 1939 the last efforts by Germany to secure Danzig had failed, which is when Britain and France came out with their defense guarantees for Poland. At that point plans were made to prepare for war, as all options had effectively been exhausted. So it is unlikely that there was a plan for after for further negotiated expansion, as Poland was uninterested in doing anything of the sort with the Germans, and they were the next target, and unlike the Czechoslovaks, they had a fairly large military and large number of alliances with major and mid-level powers.