What if Conrad III of Germany sent his army of over 20,000 crusaders in 1147 by way of an Adriatic Port like Aquileia and a large number of their camp followers stayed in the city as a logistics base and supply center? Over time as other crusades occur and German merchant traders move to the city it develops a German ethnic and linguistic majority. I'm not sure as to how plausible this scenario is.
If one is willing to ignore the copious butterflies that would certainly arise we can skip ahead a few years and see how the city fared during the 20th century. Having been returned to Prussia (Prussia having acquired the city during the Seven Years War after it was occupied by the RN) during the Congress of Vienna, Aquileia, now Konradsburg, grew into a thriving Mediterranean port. The French attempt to seize the city during the Franco-Prussian War ended badly thanks to a handful of ironclads recently purchased from the United States. Upon the formation of the German Empire Aquileia entered the Empire as an independent city.
Following World War I the city was occupied by Italy for a period of 15 years until returned to Germany by Mussolini as part of the German-Italian Alliance. Surprisingly, the referendum confirming this transfer passed by only the narrowest of margins as Konradsburg's large Jewish minority (numbering upwards of 16%) voted overwhelmingly against being part of the Third Reich. Because of this as well as being surrounded by Italy and Croatia, the anti-Jewish programs were felt to a lesser degree than in Germany proper. Still the population suffered under the Nazis and soon developed a large Army presence.
Once war began anti-Nazi elements within the city soon made contact with British intelligence agents. Critical in this was Col. Hans Oster who had been made head of Abwehr operations within Konradsburg. The transfer of Infantry Regiment 9 to Konradsburg as part of that city's defenses following the failed attempt to take Malta in September of 1940 ensured that the city would remain a hot bed of anti-Nazi resistance. Konradsburg also became one of the primary points of escape for Jews leaving Nazi dominated Europe. In early July, less than a month prior to the failed July 20th Plot to kill Hitler, (and shortly after the majority of Wehrmacht units were transferred out of the city to fight in France or further down the boot of Italy) the Mayor declared the port an open city. The United States and Britain dropped airborne troops into the city to protect it and followed it up with the largest US Marine operation to occur in the European Theater.
The occupation of Konradsburg allowed Allied forces to threaten the German rear in Italy and force a hasty withdraw. Using Konradsburg as bridgehead US and British forces were able to bag over 100,000 German troops in the Ferrara Pocket. When Germany surrendered on March 15, 1945 British and American forces remained in Konradsburg and would do so for a further 50 years. Konradsburg, despite Stalin's repeated demands to have the city divided into occupation zones, remained united, but would not join West Germany until 1954. Two years later long time resident of Konradsburg, Henning von Tresckow, would become West Germany's second Chancellor and remain so until late 1963. His assassination by communist extremists would sour Germans towards leftism for years to come. And his death was so widely felt within the greater German community as to provoke violent protests and uprisings in East Germany that would last for months.
Today Konradsburg is an independent city within Germany. With a population just under 1 million it is the only part of Germany to retain a large Jewish minority. It also retains a large military presence in the form of Germany's second largest naval base as well as being the primary foreign port of call for the US's 6th Fleet. Known as Germany's Gateway to the Far East, Konradsburg retains close cultural, historical and economic ties to Turkey, Israel and to a lesser extent Morocco and Egypt (these ties dating back to Prussian involvement in the Suez Canal Company). More derisively Konradsburg is also known as Germany's Gateway to Americanism. Besides the 6th Fleet, Konradsburg is also home to EuroDisney, several movie studies with ties to Hollywood, the European headquarters to over two dozen american transnationals along with an irritating habit of voting for candidates known for harboring strong pro-American political leanings. Adding to this is the fact that Konradsburg's city charter, which was written by a joint German-American council as part of the denazification process, contains provisions such as the right to keep and bear arms that are distinctly American in origin and cause European liberals much anxiety. Unfortunately these "special administrative functions" were written into the treaty that brought Konradsburg into the BRD and as of yet all local referendums to terminate them have gone down to defeat by wide margins.
Just an idea I wrote up. The lack of butterflies is pretty unrealistic but I've seen worse.
Benjamin