The Austro-Hungarian Empire is a formable located in Central Europe. It was a dual monarchy formed in 1867 and collapsed in 1918 for losing in WW1.
To reform it you need the core cities of Austria, Hungary, Czechia, Slovakia, and parts of Ukraine, Romania, and Poland. This can be formed if you can form a union with Hungary or Austria, (depends on what country you are playing.),
This would be an easy formable if the map was a modern day scenario, but in the 1440 scenario it may be harder.