Warning: gaminess level = MAXIMUM. If you're the type that avoids in-game exploits and cheesy tactics, this is definitely not for you.
Now, it's generally agreed upon that the most overpowered character at 867 is The Magyars. Naturally, you'd think that would be the best choice for an exploitative WC -- but that's not true. That's mainly because the Magyars start off at king-tier, and as experienced players know, is actually a bad thing.
In my opinion Nantes is the most versatile, strong, and awesome character in 867.
Look at those yummy stats, 1000 prestige, 500 gold (before divorce), nearly 3000 event troops, and 40 event boats (!). The world is your oyster, and in this case the oyster is ready to be broken.
Also, notice that he has Organizer? That's no coincidence. That's one huge advantage that Nantes has over Magyars; he begins with a randomized military trait. So I start scummed until I got organizer, which took around 1-2 dozen tries. That's seems like a lot of effort for a minor benefit no, but no, Organizer makes an enormous difference. It in fact continued making an enormous difference up to the last day of the game!
Now it's time to begin. Before we do anything, we have to do something about our culture.
Swear fealty -> change culture -> push independence faction -> no war because of your huge troop count. There you go, we are now Turkish! This gives us access to the best CB in the game, the awesome horse archer retinue, and leaves us Norse. Unlike Tengri, Norse get +50% retinue size (!) and river travel, making it by far the best religion in the game.
Now, the first move for most players is to use a Prepared Invasion on something like Brittany. We're going to completely ignore that and instead subjugate Denmark.
Awesome, we are now a Duke! In fact, we are going to remain a duke until we conquer every single pagan outside West Africa! That's due to a key strategy in this playthrough: instead of slowly and painfully holy-warring the pagans, we're going to subjugate them all, right now. Now you might be going "maxirage, that's silly! You just used up your one-in-a lifetime subjugation and can only use the king ambition to get the rest of Denmark!". Well, funny thing is, the "become king" ambition is based on your capital, and you can't change your capital with the ambition selected. Or can you?
Yep, you can easily change your capital, as many times as you want, by giving away the county to a vassal. That means you can continually move it around and subjugate EVERYONE. If you try and do this, though, you come across a problem: your vassals will hate you and will independence faction your butt off. But you know what they say: "they can't join a faction if they're in prison!" (they say that, right?). So we're going to imprison and banish every single vassal we acquire, down to the baron level. On top of cleanliness, this will give us an enormous amount of money to hire mercenaries and subjugate more people even faster. In this playthrough, tyranny is completely ignored; you're pretty much going to be the worst dude on the face of the planet. But it doesn't matter, because imprisoned vassals don't rebel!
Imprisoning all these vassals is fairly luck-based, though. Can't we make it easier?
There you go! Create bishop -> give him county -> plot to revoke it -> imprison -> banish -> gain 1 intrigue. This also generates a net 5 piety, meaning you have access to both infinite intrigue and infinite piety! I said this is going to be really exploitative, right? The only limit here is tediousness. Just use your martial to suppress revolt, which increases the arrest chance. Combine it with 90+ intrigue and you'll almost always get 100% arrest chance. Fire and re-hire your martial to move him around as much as you want to!
Once you subjugate Cumania, it comes to an end. You become a king and cannot use the ambition any longer. At that point, it's time to use the good old Tribal Invasion CB on the Christians and Muslims. The key is, instead of fully occupying the country, do the bare minimum for 100% warscore. Then imprison and banish whoever is left! Together with the boats scored from Scandanavia, the ability to traverse rivers, and the massive amount of mercs and retinue, you can conquer everything in lightning speed.
The limit here is troop movement. That's why Norse is the best -- moving on water is much faster than land, and boat bombing downright breaks the difficulty curve. But what about inland areas? In that case Organizer truly shines. First off, the speed bonus is modified by your martial stat, and Nantes starts with 24 martial! Do you hate it when a smaller army avoids your larger one by leaving one day after you arrive? With organizer, that will never happen. On top of that, it will allow you to reach water-free provinces such as West Africa as quickly as possible. There are no words to how much time and trouble this trait saves.
Then some tedium later (scratch that, a LOT of tedium), you'll be on the road to WC. By the way, it's actually impossible to go fully vassal-less, as the game literally can't handle it and will hang on you. You should avoid a demesne size of above 900 just to spare your sanity. Also, you will need to make some vassals to handle rebels (which is actually the first time I've ever used levies in this playthrough). Use 3-4 superduke vassals, and then imprison them. When you raise their levies, their entire realm's forces (20% due to crown authority) will be raised in the single province you selected. You can essentially teleport your levies all over your realm.
And there you go. After cheesing the heck out of it, and a bunch of work, you'll conquer the world.
PS: looking at the 2.0 patch notes, doing this strategy should still be possible. The biggest problem would be the nerfed boat bombs!