Probably the Free Territory of Trieste as it existed in OTL
https://en.wikipedia.org/wiki/Free_Territory_of_Trieste --with a Zone A administered by the Western Allies and a Zone B by Yugoslavia--was always fated to be divided between Italy and Yugoslavia. Yet economically, some sort of Free Territory status made more sense--at least in pre-EU days (and maybe
post-EU days in the future!)--than Trieste being part of Italy. The economic function of Trieste is to be a port for the nations of east central Europe, as it once was for the Austro-Hungarian Empire. Italy has better ports for its own goods.
Indeed, for a while in the early 1990's, the issue of irredentism versus Trieste as free trade port for east central Europe divided the Northern League in Trieste from the local neo-fascist MSI:
"I got a chance to see the League's competition in Trieste up close when I stopped by the MSI offices as December's local election results came in. On the wall were posters calling for a "new irredentism," illustrated with pen-and-ink sketches of Dalmatian cities -- Fiume (now Rijeka), Zara (Zadar).
"Roberto Menia, a national coordinator for MSI, openly said the party would take advantage of the relative weakness of Slovenia and Croatia to redraw the borders. "We have struggled for 50 years," he said, 'and now that we are stronger we will renegotiate everything, borders included. We want the territories we had before the war." If necessary, the party would cooperate with the Serbs, Menia said, pointing out that his party's president had already visited Belgrade. "We know that our own interests and the interests of the Serbs can fit together."
"With that sort of memory in the air, the League in Trieste concentrates on the future. "They (the MSI) want to put our Italian flag on Istria, Fiume and Dalmatia again. I think that's a nineteenth century way to think," Fabrizio Belloni, head of the League's office in Trieste at the time of my visit, said. "If I were a solider and I got back Istria for Italy there would not be one more ship in our gulf."
"Instead, Belloni pointed to the future Trieste could have if it opened up to the east, as a free trade zone, rather than bickered with ex-communist countries about borders. "We want to give the city the legal status, financial power, economic power, and technology to be what history and geography made of Trieste -- an open gate between northern Italy and the Eastern peoples now freed from Communist slavery."
"The only historical period to which Belloni harkened back was Trieste's period as an Austro-Hungarian port. "We feel that we are a component of Mitteleuropa," he said. Better Austria than the (discredited Italian) Christian Democrats, that's for sure."
"And Belloni had no time for the politics of paranoia about the use of Slovene or Italian on television stations and road signs -- issues which the neofascists had played up. "I have a postcard of Trieste from just before the end of the Habsburg Empire," he said. "On the back the word 'sender' appears in eight different languages."
"Belloni leaned back in his chair and smiled. "Now
that is civility.""
http://www.icwa.org/wp-content/uploads/2015/09/CRR-15.pdf