Personally, I'd like to see someone like David Yates or Matt Reeves doing it. Reeves is frontrunner for The Batman and Yates is busy with Fantastic Beasts, but that's the kind of man I'd get for the job. Both of them can do the sort of "Dark" story people seem to crave in superheromovies that aren't MCU, and both have incredible visual styles.
Of course, I'd hope it wasn't a Fan4stic 2 but a reboot done right, whether in the MCU or not. I'd also like it to be a movie presenting the 4 as already established - i.e. no origin story, but they are already heroes at the get-go.