Harry, Ron, and Hermione
So I have this idea that after their kids graduate from school and are off on their own, the trio get pretty restless and stir crazy, and work as vigilantes, hunting down dark wizards, working outside the Ministry's jurisdiction.
I mean, you remember what nosy, interfering little shits they all were as kids, there's no way they would be content with a calm adulthood. Grindelwald came less then a century before Voldemort, so surely there will be another one.
Also, I can't decide if Ginny has taken over Madam Hooch's job, or if she is now a partner in Weasley's Wizard Wheezes.