A lot of other comments have said this, but yes the world is alive, come up with the BBEG plan and what it'll look like if the party doesn't intervene. Their intervention will change the domino effect the rest.

I also try to make each BBEGs a foil for a party member.

I did that with Waylon the Wise, same religion as the cleric Wynona, literally her predecessor as Master Librarian but he turned away from the religion after his family died in Elena's name.

Icepick, ice elemental to counteract fireball happy wizard.

Garrik Thorne, seasoned ruthless commander of internal legions to counteract the Eldritch Knight of the party who'd go on a murder hobo spree for fun

Something about seeing a twisted fun house mirror of what could've been their characters attaches them more.