Empire of Germania:
Started in 769 as king of Bavaria, promptly holy warred my way east ideally jumping on 1 province nations as you can 100% them by easily even if others join. Transitioned to retinues, only bothered raising levies for big wars.
The infidels took all of France and so I outgrew them by taking more pagan land, when i felt big enough started chiping into France with holy wars, then pushed them even further into iberia. I bred a strong dynasty with lots of geniuses and strong children and one ruler who was both and gay. I like all my titles going to my youngest because it gives me multiple chances to make a decent heir, one of my heirs was terrible so i went on a seducing streak and legitimised a genius bastard which more than paid off for the -1 from being a bastard.
One thing i wish i had realised earlier in the game is how much you can milk the pope for. i mean i know its based on my own income but every new pope i make him like me and get 3k gold off him easily, im now using it to build cities all over the pagan lands, my dynasty also control the teutonic knights (who are also my vassal). Im running absolute crown authority so have just shy of 100k levy troops on top of the 50k troops i have from skirmisher retinues. I've capped out on my realm size until i pass imperialism for the extra vassals, I've transferred all the vassals i can and given out all the kingdom titles i control to reduce my vassal count but unfortunately a couple of kingdoms drifted into mine and for whatever reason the game wont let me give it to one of the original de jure vassals because it is now a titular title.
I created 6 vassal republics on different coastlines to fill the seas with trade posts and as a result they used to be my highest paying vassals untill I had to transfer them onto their local de jure kings to reduce my vassal limit. I mean I guess I can keep expanding but I would have to make my existing vassal kings more powerful, I havnt had any faction trouble all game because i have always ensured I was stronger than my vassals and Im ery reluctant to give any of them more power.