Poland finds itself in a very strong position in terms of total development at the beginning of the game to dominate whole Europe and the world. Having Lithuania as a subject that eventually will be integrated into the Commonwealth is another strong argument to that.

Geographically it isn't as well protected as it could be, and therefore we need to ensure that our southern and eastern borders are taken care of.

Start with weakining Ottomans by taking Constantinople/subjugating Byzantium, and therefore preventing them from using their jumpstart into expansion. Then use the dividedness of your eastern neighbours and their initial dependence on Golden Horde to cripple them even more and prevent from unification to secure this flank as well.

This chain deserves to be extended with more steps and maybe also chained together with Victoria 3 and HOI.