Sorry, rant incoming!
Conquer Kingdoms by Force
- Utilize the "Prepared Invasion" if you meet all the requirements (it's are pretty restrictive and judging by what information you've given us, this is most likely not an alternative).
- Ask the Pope if you're Catholic; Sanctified Invasion and conquer by divine right.
- (
Unsafe) If you're Nordic, grab the counties of Uppsala, Naumadal,Sjælland, Nordurland Vestra (Northwestern Iceland) and Braunschweig, reform the religion and pray that Sweden and Norway doesn't convert (grants you Holy War* against heretical nords).
- If Sweden and Norway has converted to Catholicism/Chalcedonian, follow the example above and crush them with Holy Wars* and conquests (there's a long cooldown on Holy Wars* in CK2+ however, not as simple as in Vanilla).
- Conquer a Slavic or Baltic region, convert to one of their pagan religions and start your slow "Tile by Tile Conquest"; it will take time, especially a whole kingdom. But if there's no other way out, try it. And when you're done, if you really want to remain Nordic, just move your capital again convert yet again. Gameified as hell, but it works.
- (
Unsafe) Reform another pagan religion, beg that the idiot church declares Holy War* on the right kingdoms (seriously however, this only works with the Nordic fate; as you are the "Not Pope" and you yourself declare the Great Holy Wars*).
- (
Unsafe) Truce Break like crazy. Just continuously waste your Prestige, break every damn oath you've ever sworn and ignore the losses. To keep up your Prestige, raid Rome, Venice, other republics and theocracies constantly, conquer Finland by conquest, conquer Lithuania, Poland, Novgorod, Perm, Muscovy and Kiev using the same method. There's a high risk that your kingdom spirals out of control doing this, but you might be able to cut a large piece of Sweden or Norway doing this, then just grant independence to the crap-lands you don't wish to keep. It should be noted, that this is one of the most hilarious ways of "fucking up your game", as chaos will ensue, trust me.
- (
Unsafe) If you don't have the Horse Lords DLC (I don't, so I can't say anything about it), try reaching
Altaic territories and embrace one of their cultures. Preferably Turkic, as this gives you access to the slightly overpowered "Tribal Invasion". A Casus Belli that allows you to conquer just about anything none-pagan (Christians, for example), just remember to read up on how it works, it's "odd" the first time you use it (more info
here).
Conquest by Marriage to a Princess
Simply marry a daughter of the Swedish or Norwegian king. This requires that you don't have any land within Sweden or Norway to work however, as they won't accept the marriage due to "Political Concerns". This gives your heirs (sons and daughter of the princess, not all heirs; ignore concubines and lover while doing this!) claims on the kingdom once the mother (princess) dies. It takes time, but works just fine when you're not in a rush.
Remember, you've to claim your throne as the heir of a princess/prince; war is necessary in the end. Unless the enemy dynasty is very small and you're able to assassinate the rest of the pretenders (all of them).
"Conquest" by Marriage & Inheritance
There are a lot of cheap-shot ways to conquer whole empires with marriage. For example, if a kingdom has the gender law "Agnatic-Cognatic" or "Absolute Cognatic", then it's possible to "force" a Queen upon the throne. How? Assassinate every son the king produces. Constantly raid the kingdom, sack their towns and execute them if you imprison them; constantly plague them.
If you're in luck, a woman will inherit the throne and become Queen. Why is this a good thing? The AI for women in CK2 is sadly piss poor, to put it lightly. A Queen will more or less give a way her kingdom as she won't exclude regular marriage. She'll happily enter a regular marriage with your king and your heirs will lay claim to both kingdoms.
This tactic is extremely volatile however. If the Queen dies before you, your idiot son will rule an entire kingdom by himself, most likely messing it up and when you finally die, you'll have to clean up the mess. This usually takes decades if you're in bad luck. But hey, the kingdom is yours. Killing shitty heirs, divorcing the ugly lowborn wife your idiot son married when his genius Princess of the Holy Roman Empire died, fixing border gore and worst of all, revoking and clearing up the duchies in the kingdom, that your idiot son gave to the inbred, adventurous Prince of France.
Also known as: a lot of headache.
Conquest by Fabricating Claims
In CK2+, it's more or less impossible to abuse "Fabricate Claim". It's expensive (~200 per claim, really harsh as a pagan) and unless you're feudal or a merchant republic, you won't be making the gold required. Of course, raiding is possible, but that means your army is on the move and the AI has a tendency of noticing it. In Vanilla, it's fully possible to abuse the fabricate claims mechanic. If you willingly put a child (the younger the better) on the throne (example, get depressed and commit suicide), you'll have about 50 years (my kings and queens usually die at this age, give or take) to fabricate an entire kingdom. And when you've all the claims you seek, press them all at once and bathe in glory. Not worth a whole queens or kings life however.
It's not a safe tactic either. I do it from every now and then, especially in Vanilla. But when your king or queen dies at the age of 30 because of poor health, you've wasted a shit-metric-ton of gold and the claims are lost.
Off Topic
The marriage/inheritance tactic works well. In one of my most memorable games I conquered the whole British Isles as the welsh family Alt Clut of Strathclyde (Charlemagne starting date), more or less used this tactic exclusively (war happened, of course). Turned my gaze to the Byzantine Empire, got a Queen on the throne, married her. Son gained control of both Prywdian (Britannia) and Byzantium. Reformed the Roman Empire after some warmongering, or the "Welsh-Roman Empire" I guess. Converted to a christian heresy (Fraticelli), usurped the Holy See for the one true Pope, almost reconquered the Roman borders, but got bored before I finished it (blobbing gets boring in the long run).
*PS
Holy Wars are not possible if the crusades hasn't begun
