[Varyon]

There are plenty of options, but don't put too much stock in whatever your plans are - the players may well throw you a curveball that you'd rather pursue. That said, here are some options:

Always a Bigger Fish: This runs the risk of power creep, but basically while the Big Bad was the top of their own heap, there are worse entities out there... and defeating the Big Bad has brought the party to their attention. Or for a "You fools, you know not what you have done!" / Good Job Breaking It Hero situation, the Big Bad was actually holding something back (and/or trying to gather power to defeat it), and now the heroes are left holding the bag to deal with it.

Power Vacuum: As already noted, taking out the dude on top typically doesn't suddenly mean all their underlings stop being an issue. You could potentially combine it with the above, with the Big Bad having served in part to hold the underlings back from going full-on goblin mode. You could also have the internal struggles as someone tries to take the Big Bad's place cause serious issues for innocents. Or you could go for a "Scouring of the Shire" incident (or several of them) - former lieutenants of the Big Bad splinter off their own small groups and basically set themselves up as warlords (or whatever the equivalent is for your scenario), each dominating a small area (possibly one connected to the PC's), and now the PC's need to deal with them.

Retired Heroes: This really can just be the end of the story. They've won, they get their just rewards, and go off to live happily ever after. You can, of course, have a later, mostly - or even completely - unrelated adventure in the same setting; having it occur a generation or two later, with the old PC's as NPC supporting cast, would be a good option.