A lot of fans were excited about the idea that Quentin Tarantino will start a Star Trek spinoff with R rating and then retire after his tenth movie. however this will probably not happen, because Tarantino recently confirmed to Consequence of Sound that he hasn't been in contact with the studio management for a long time now. After Once Upon A Time In Hollywood, with a top-class cast and a budget of 90 million dollars, you can also imagine that Tarantino might be able to slow down a bit and take on a smaller, more intimate project as a conclusion to his legacy.
He himself says: that there's a really big idea in his head. ...but right now, the idea of a smaller audience just feels right and appealing to him."
So the intended horror movie also might be pushed into the distant future. In the summer, he said that he could imagine a horror movie as the last movie, if he had an idea. Nevertheless we will have to be patient for quite a while until Tarantino has finally decided on his final movie.