Zack Snyder's Justice League created a litany of hype around the DC Cinematic Universe, revitalizing interest in the controversial cinematic universe. One particularly popular element of the film was the presence of Darkseid and the New Gods. The DC Universe's ultimate villain may have only had a few scenes, but his presence was still felt throughout this vastly superior version of the film.
But however powerful Uxas of Apokolips may prove to be, there's another DCEU villain who could ultimately prove to be even greater. Doomsday was supposedly killed in Batman v Superman: Dawn of Justice, but his comic book counterpart is known for his longevity. If the DCEU Doomsday is anything like the comics, he's sure to come back in one way or another.
Comic Book Doomsday
Doomsday was created in the comics by an alien scientist with the intention of creating the ultimate killing machine. He did so by placing the creature in its infant state on prehistoric Krypton, constantly exposing it to terminal threats. This forcibly evolved it so it was unable to die from the same thing twice, and in general, become stronger much faster.
This instilled in the creature the ability to mutate to deal with any threat, as well as an ancestral hatred of anything Kryptonian. Thus, when the full matured beast came to Earth and encountered Superman, he fought the Last Son of Krypton in a battle to the mutual death. And Doomsday would continue to evolve, to the point where even Superman wouldn't be able to take him out the same way twice.
DCEU Doomsday
The cinematic version of Superman's ultimate foe was a good bit different than he was in the comics, being made from the corpse of General Zod. This mutated abomination had exaggerated versions of regular Kryptonian powers, making him extremely powerful. He still bore a resemblance to the comic book version, though, in that he mutated further to become more powerful, growing his trademark bones after taking damage.
This already hints that he could return in some capacity and cheat death like the original Doomsday, but so does Superman's return. The Man of Steel was brought back to life after being supercharged by a Mother Box, and it's entirely possible that Doomsday could be brought back in the same way. If brought back, he could also continue to mutate and evolve further to become a more efficient killing machine, as shown by his bone growths. This could even result in him gaining greater intelligence like he briefly did in the comics, or perhaps developing a distorted version of Zod's personality and memories. If he shares the comic book version's aversion to dying the same way more than once, that means Kryptonite would be absolutely useless. This would make the creature more brute force than ever and incredibly hard to defeat.
One potential storyline could involve Darkseid or one of the other New Gods using a Mother Box to resurrect the monster, with the goal of him serving the ruler of Apokolips. If Doomsday ends up rebelling against Darkseid, this could result in a sort of adaptation of Hunter/Prey, where Doomsday beat Darkseid, only for Superman to take him on later. This, of course, involves the DCEU continuing on any of the threads Snyder laid out, but it would certainly redeem what many saw as a lackluster version of Superman's killer.