Re: Does it HAVE to be a trilogy? Can it go beyond three films?
Trilogies work, unless you have a beloved book series. Even the MCU is a set of trilogies.
Superman, like Batman will gain a great deal of notoriety and success as a trilogy. If someone else wants to follow that with another trilogy, or lead that into a DCMovieU with JLA or World's Finest or Trinity or whatever, good for them.