Truest principle of RPGs. You can’t plan on the party going anywhere you expect or doing anything you expect. My solution is to design all situations, villains, etc. as drop-ins and drop them in wherever the party ends up going. The entrance to the ancient tombs of the evil priest lords could be in some ruins the middle of The Sinister Forest, or it can be under a village blacksmith’s forge in the Happy Hobbit Foothills if the party decides to detour around the Sinister Forest. Doesn’t matter, they’ll get there.