To be sure, Harry Potter had the sequel problem that the problem of the books was explicitly portrayed as larger than any other, leading to issues of anticlimax when trying to go on. But it did have Grindelwald, which raised the possibility of other evil wizards.
To set up for a sequel -- if one is wanted, and if you know it enough in advance -- there might have been openings for planting more history. Such as, while trying to claim that Voldemort didn't come back, the Ministry could claim it was an absurd as this, that, or the other wizard, of whom it was also said that he came back, but it had been proven false. Or allusions to past fights against past evil wizards. Or even mentions in history class.
The fun part is that you don't need to use any of those for the sequel if the world is convincing enough that the reader would believe that another evil wizard arose.
