I found a rather sneaky way to become very powerful within a mere several turns with the Byz.

Simply take your Emmisary and go around all of the many rebel provinces to the North and bribe away. Most of them have very little loyalty and are very cheap to bribe. Within 15 or so turns, you'll have as many provinces.

Notes:

- If you like, when your emissary is finished bribing the final rebel provinces around Muscovy, have him take a brief journy through Germany and Denmark to bribe Norway and Scandanavia (there's a good chance that your emissary will be assassinated when passing through those territories, though).

- There is not much fear of this causing you to go bankrupt, because as you bribe more and more provinces, you will be getting an income from all of them.

- This strategy is possible with the Turkish, also, but not advisable. The Turks would have to deal with a lot of revolts since they are Muslim. Most, if not all, of the provinces in question are Orthodox, as is The Byz.