As far as filming a new set of Prequels, right now or in the next couple of years, it will never happen. 15-20 years from now, maybe.
Could they shoot additional footage and add to the Prequels to make them better, like some of the horrendous dialogue, yeah, they could do that. Will this happen, 100% sure it won’t, at least as long as George Lucas is alive, it won’t. There might even be a clause in the contract that Disney’s Bob Iger and George Lucas signed saying they will not touch certain or all films by making changes to them, who knows really.
Kathleen Kennedy will NEVER let it happen.
Currently, there are 10 Star Wars films, and 12 if you count the Ewok films. The Last Jedi and the Han Solo film are just about done, it will only take editing the footage and it’s done.