Just had an interesting start. I started as the Duke of Moray using a custom ruler who I set as Welsh, with an eventual goal of spreading Welsh culture across the British Welsh Isles.
First thing I do is marry Margaret of Wessex. Then I jumped on the count of Buchan on a de jure claim, then plotted (successfully) to revoke his title giving me a third county. During that short war the wife got pregnant and had a son right after it was over. I immediately plotted to kill the wife, which my courtiers were eager to back up. One successful plot later and my son had a claim on England and I was marrying a younger, smarter, woman.
I declared war on England to put my two week old son on the throne, hired mercenaries, and set off with 2000 men and 20 ships against England & the HRE on one side, Norway moving in on Northern England, and Normandy moving up from the south. I stayed coastal using hit and run tactics on the provinces that had recently been taken by Norway and Normandy, retaking them while they had small garrisons and fleeing to the ships if one of the huge armies approached. I also opportunistically stomped a few small (1k and below) armies that I spotted along the way, paying my mercenaries with the ransoms of the nobles I captured.
About five years into that the King of Scotland had forced the Duke of the Isles to join Scotland, which gave him a Duke that wasn't too happy with him. I immediately plotted to take the throne, and the new Duke was happy to join the plot. Within a week I sent the message to the King of Scotland, and he did something rather strange. He gave up the throne, then immediately revolted for his independence - with the count of Fife joining him after a few weeks. The Scottish army, now under my control, sieged and took over the former King's county - and once the war was over I revoked both his and the count of Fife's titles since they were traitors. I then recruited Fife's men for my own uses and moved that army, which was all total another 3200 men, down to join my group of now 1800 men in England.
Two more years of sieging, this time with lots of storming the castles with my larger force, and I had the King of England at 90% war score. And I had a revolt on my main province. Only 700 men, but I really needed my guys to finish off England while I had the chance - so I ignored them. By the time I had England at 100% war score they had taken the county, and RIGHT before I finished the war I suddenly got the event where one of your courtiers gets maimed by rebels - and of course it's the 7 year old son who is about to be King of England. I put my maimed son on the throne, then joined him in his wars against Normandy and Norway. With my 5k men and his 2k men we crushed both of the 5k stacks that were left of Norway and Normandy, and both of them accepted white peace. Less than a month later the son dies due to being maimed, and I inherit the throne. I had planned on letting the son keep it until he inherited Scotland, but it ended up the other way.
So there I was, ten years into the game at 1076, from Duke of Moray to King of England and Scotland. The vassals didn't seem to like this, and the big English dukes revolted. Luckily I still had the 5k stack in Moray after putting down the rebels, and another 3k English troops soon joined them. Over the next few years I stomped the rebels and imprisoned a few more before they could revolt, soon deciding on just imprisoning all the rebellious bastards. Within four years I had every single one of my vassals imprisoned, mostly during war, and I quickly imprisoned all of their vassals too via the option (they couldn't even revolt when I failed).
So what was I to do. All of my vassals hated me. The Pope hated me (imprisoning is tyrranical!). Well, If I'm a tyrant I might as well act like one. I checked my piety and checked the wealth of my vassals, realizing I had enough piety to banish most, but not all, of them. I banished all the wealthy ones, and stripped the titles from the poor ones. I was soon left with almost all of Great Britain in my personal demesne, and about 3500 gold in my coffers from all the confiscated wealth from the banished nobles. I picked a good sized personal demesne across the two wealthiest duchies and a few spots that hadn't had the ducal title formed yet (I had a super wife, so I could hold 12 lands) and revoked all the titles of the baron level vassals there, too.
Then I began inviting nobles to my court. It scales to income, and with the massive demesne penalties my income was zilch - so they only costed 1 gold each. Soon my court was overflowing with capable Welsh rulers (all with 13+ stewardship), who all really liked me and knew nothing of my tyrannical ways. I divided the land up with a few counts and a duke over them in every duchy, and soon all I had left was my personal demesne. I picked the remaining good rulers to manage my cities and extra castles and a few churches and then just randomly created the rest of the church rulers.
So here I am, year 1080, age 30, ruler of an entirely Welsh owned England and Scotland. All of my vassals absolutely love me. I have a 300 piety from giving back the church titles to new owners. I have over 3000 gold left over from robbing all the nobles. I still have 1200 prestige from winning all the wars. And to top it all off, my 10 year old heir inherited both positive traits from my new wife - Genius and Attractive - and he already has 15 stewardship even without an educational trait so he'll definitely be able to manage my big demesne. And half my duke vassals are Celibate (the game likes generating celibate nobles for some reason) so when they die off I'll be able to assign the lands to my dynasty if I want. The only downside is the Pope absolutely despises me for being a tyrant, so I'll probably be excommunicated soon.
Still, I don't think I've ever had a game where things have gone this good for me. If this luck keeps up the Pope will suddenly die off and be replaced with one that loves me before I get excommunicated.