The below is one of many player suggested strategies for Bulgaria. Bear in mind, due to the dynamic nature of the game, it may unfold differently for other players.
Bulgaria starts under Ottoman rule, release the nation and play as vassal. While a vassal improve relations with Ottomansrivals and try to get them to support your independence. After you get help, declare war and group up with your allies's armies. Finish the war and gain independence. When you are free you can complete a mission to gain cores on the surrounding areas around you.
When you gain independence you get cores on Rascia, Serbia (area) and Macedonia, by the time you are independent Venice probably got territories from Bosnia or Serbia, while they're exhausted by the war you can attack them. You could also attack Wallachia If they're still free.
Once Bulgaria has established itself by expanding and stabilizing you could go for the Caucasus , after that you can try to destroy the Ottomans with the help of your allies or go for the Pontic Steppe.