Kingdom of Slovakia in all its glory.
First of all, this is impossible for anyone except players, as it requires to make titular duchy your main title and then create custom kingdom out of it...I am not even sure if they can create custom kingdoms. This required many attempts during which I managed to die on pilgrimage, get my army killed by smaller army and so on. Luckily, this all happened in first 5 years of gameplay, so it all took me maybe an hour before I found out what to do and started playing properly.
So, I started in 1066 as duke of Nyitra, who is also duke of titular Duchy of Slovakia and also heir to Duchy of Ungvar and Duchy of Transylvania. And also has claim on Hungary. What happened in all failed attempts and also in succesful attempt was that my brother started faction for elective succession and king always accepted his demands. I managed to put myself as heir and then bribed everyone and killed the king. So, I became king of Hungary, which, as I found out, ruined everything. I suddenly had de jure kingdom and i couldn´t give it up to create titular kingdom from titular Duchy of Slovakia. So I decided it is time for my brother to get to throne through faction. Firstly, I managed to kill my another brother, so I ended with duchies of Ungvár, Nyitra and also duchies of Esztergom and Balaton, which belonged to former king. Plus, I had Duchy of Slovakia, so I ended up with 5 duchies. Kind of cool. Problem was that my remaining brother couldn´t gather enough power to demand my abdication. So, I sent small armies to peasant revolt, to reduce number of my soldiers, so he would get bigger percentae. He was on 130% and that coward did nothing. Finally, I died in one battle against peasants and was succeeded by my infant son (who was, of course, educated by duke of Bohemia...there is no Slovak culture, so I needed some other West Slavic guy to educate my son properly). Finally, my now uncle fired his faction and I gladly accepted his demands. This made him king with 1 duchy, 1 other vassal duke and 2 vassal counts outside of his duchy. I got remaining 4 duchies. I waited until I become adult, gathered money, replenished levies and then started independence faction, which I got to more than 100% all by myself. I fired it, war started and I won. I switched my main title to Duchy of Slovakia and created custom kingdom.
Now, after 100 years, Kingdom of Slovakia is de jure kingdom with 2 duchies (3rd will become part of it in 4 years) and 3 other duchies drifting into it.
During this game, few funny things happened. Firstly, Duke of Bohemia was really bad educator and managed to get future king Filip I. maimed. Luckily, king Filip I. lived more than 60 years and outlived even his oldest daughter (his second daughter was forced to become a nun, while his third was executed in his prison...gavelkind makes you do bad stuff...just like killing 2 youngest sons of your oldest daughter, so just 2 older remain), his oldest grandson, only to be succeeded by his last remaining grandson. So, yes, 66 years old guy, who was maimed aproximately for 60 years and still lived.
Also, what is really interesting is that Duchy of Slovakia is not de jure part of anything. Only example of this that comes to my mind is Papacy (and maybe Ecumenical Patriarchate, although you don´t see that being independent really much). What does it mean? If you have Duchy of Slovakia as your main title and you create custom kingdom, that custom kingdom won´t be de jure part of any empire. So, in my game, there are 2 empty spots on map of de jure empires: Duchy of Latium (pope got to keep it all under papacy for 100 years) and duchies of Balaton and Ostrihom (Esztergom), which are current de jure parts of Slovakia. So, it will be interesting to see if someone creates Carpathia or Wendish empire, as those are lands that are currently becoming smaller with growth of Slovakia.