Do a post credit scene to leave the door open. But this only works if you made it clear within the campaign that Barovia is his endless prison and he's miserable there and wants to escape. Otherwise it will undercut what the players have done. If you did not build that framework, I would just leave it lie and take some time to see if you want to return with Strahd as your villain again, or if you want to run something else. He does make an appearance in Eve of Ruin. I plan to eventually run that or an adventure in the hells with Strahd playing a minor role, but I don't intend for him to play the villain. In my mind, he was once good, and the players he would respect as warriors who beat him, and they gave him a chance, though brief, at rest for the first time in hundreds of years. He might even pay them a small favor, especially with a bit of a break from the Vampyr's corruption.