If you've not tried playing Croatia with 769 start yet, I suggest you try it out.
Since I am myself from Croatia it was the first game I played. Since then I played plenty of other factions, but non so far offered the same potential for expansion from almost nothing.
Try to defeat the other Croatian duke ASAP, one alliance should suffice to give you that extra kick.
If you can manage to fight off Avaria and Hungary and get most of your De-Jure territories you can become strong enough to start gobbling up your neighbors to the east. Italian and Byzantine provinces give you a pretty fat, juicy target for raiding, providing 1 gold every 4 days.
Key is to convert to Christianity in the fist 50 years, it needs to be timed just right. Then you can go ahead and purge your lands of the pagan lords and grab their lands for yourself.
Now you have a very strong tribal blob combined with holy war casus belli for every neighbor around you. Just wait until they are distracted by something else and snatch a few juicy provinces off them. This way you can blob out really fast. You will know when it's time to convert to Feudalism. I was able to do it before 867.
By the time crusades start you should have land not too far from the target areas, so you can safely keep the conquered land to yourself, and make a pretty fat profit doing it.
Eventually 4th crusade will destroy Byzantium leavling it's remaining lands ripe for your picking, then it comes down to taking all the muslim lands to the south. Make sure you don't expand too far to the east so you won't become a bulkwark against the mongol horde. They should fall apart into small realms eventually, leaving you with another set of easy pickings.
I've yet to find another faction that offers the same opportunities for expansion while also being able to catch up with major European powers in record time. And at the same time isn't also constantly picked on by numerous neighbors.