Here's some ideas I had for Superman going forward in the DCEU, after Justice League.
The next solo MOS film should have Brainiac as the bad guy. A cosmic, sci-fi heavy adventure mixed with personal stakes with Clark and Lois upping their relationship (maybe a marriage, seeing as how he was about to propose before the events of BvS).
His rivalry with his archnemesis can't be forgotten, however. I have a feeling the events of BvS and JL will shape Luthor into more of the controlled and patient mastermind we've seen in other stories. As such, with his knowledge of Superman's secrets Luthor could be an overarching villain over more than one film, in MOS 3 Luthor can create Metallo as an exploitation of the kryptonite that nearly killed Superman in BvS
Finally, an adaptation of the Public Enemies storyline would bring Superman and Luthor's rivalry to a head. Luthor obviously won't be president in the DCEU after being exposed the way he was in BvS, so perhaps he could blackmail the president or find ways to brainwash him to do his dirty work. In a mix between Public Enemies and the recent Rebirth arc where Luthor tries to become a new Superman, Luthor could build himself a battlesuit and try to take Superman's place while Superman is turned into a fugitive.
Then we would finally get a no-holds-barred showdown between the two arch-foes.