The Republic of Liberland.

"The Republic of Liberland" is amusing as a protest/joke/performance art. But as often happens with these perennial micronation attempts, people are taking it too seriously by getting arrested over it, or arguing over what serious policy decisions their fictional government will make. There is never going to be any internationally recognized sovereign state on that patch of muddy swamp between Croatia and Serbia. The land might be Croatian, it might be Serbian, but neither they nor any other nation will accept the assertion that it is neither. And Croatia as the de-facto owner of their side of the river, will continue to arrest people for illegally crossing the border.

If European libertarians really want something like that, I suggest Free State Liechtenstein. They are (unlike Croatia) part of the Schengen Zone (but not an EU member), so all EU citizens are free to move there without even a passport check. The population is tiny: less than 40,000. It's already a tax haven and refuge from stupid laws in the rest of the EU, with a relatively inoffensive and limited government and a liberal constitutional monarchy. And as a bonus they have actual roads and buildings and infrastructure and enough land to build more of those things on. They'll even right now rent out the whole country for parties, no joke. $70,000 a night and they'll change the street signs for you, print your face on temporary currency, and make you the sort-of head of a state for a day. Pretty mountains, too.

If you want your libertopia on the European continent, tell people to start moving there, not a sliver of a few disputed acres in the Balkans.