So first off, everything in this film happens assuming A) Superman lives through Batman V Superman and Justice League, and B) Superman is now fully recognized and beloved by the public after the events of the previously mentioned films.
The Villains:
MR MXYZPTLK: He is the most powerful being in the DC Universe, and possibly the most powerful in all of comic book history, if you can believe it. He is the definition of overpowered. But, he doesn’t use his powers for truly evil purposes, which is why I think he would be the best primary villain, as he gives off a lighter edge. His primary motivation is that he sees SUPERMAN as the most powerful being in his own universe and just wants to see who is stronger. As the film progresses MXYZPTLIK becomes darker and darker until he eventually awakens the secondary antagonist…
DOOMSDAY: His origins will stay the same as the comic for the most part, created on Krypton and has a thirst for Kryptonian blood. He will be trapped underneath the surface of the earth for many years, until MXYZPTLK awakens him (much like how Jason was awoken by Freddy in Freddy vs. Jason). He is woken because MXYZPTLK needs SUPERMAN weakened as he keeps losing in direct hand to hand combat.
The Plot: The film would open with DOOMSDAY’S creation on Krypton, murdering his creators, and jumping off screen into the next scene on Earth.
Again, assuming all of the above, SUPERMAN would be beloved by the public, and CLARK KENT would be getting along nicely with LOIS (Cliché superhero beginning). MXYZPTLK would then appear on earth, and would see news reports and plaques and statues dedicated to SUPERMAN, and smile as he has just found a worthy champion. As the movie progresses, SUPERMAN and MXYZPTLK would fight, always with either SUPERMAN or MXYZPTLK retreating. After each fight, MXYZPTLK would call the fight “round 1” then “round 2”. After he is forced to retreat during round 2, MXYZPTLK is pissed. He then goes for DOOMSDAY, who he intends to use as his “trump card”.
MXYZPTLK starts the fight in Metropolis, and then brings in DOOMSDAY. There is a fight, closely matching the match in Death of Superman, and it ends with SUPERMAN killing DOOMSDAY, but sustaining serious injury. He then goes to MXYZPTLK, who taunts SUPERMAN about losing, and tells SUPERMAN that there is only one way to get rid of him. SUPERMAN then grabs MXYZPTLK, despite being close to death, and begins beating him with the same level of intensity he fought ZOD with in Man of Steel. Scared for his life, MXYZPTLK says his own name backwards, and disappears. LOIS then comforts SUPERMAN as he dies on the ground.
I will add pictures later, but if you have any thoughts, feel free to comment, and if you like, thumbs up!