Q would have been fine for a cameo appearance, but not an entire movie around him. Tomolak or even Sela would have been better than Shizon for Nemesis. It would have also been nice to see some of the DS9 characters in a movie like Gul dukat, Garak , Quark, or any of the station crew.
How would Q only be good for a cameo but Sela, Tomalok be good to have a whole movie built around?
Q has limitless story potential, the others are one trick villains there to simply offer a stumbling block for the crew. Give me Shinzon every day of the week and twice on Sunday over Sela and Tomalok.
That's why Q is horrible for a movie. There's no way to defeat or overcome Q, so in the end he just presents situations for the crew to deal with that come out of nowhere with no way to stop them. He's good for episodes where there is some sort of lesson to learn and he is the teacher, but again, then the focus is on Q and less so the main characters. It doesn't work for a movie.