If the Justice League was allies of the Avengers then Thanos would be enemy number 1 with Darkseid. The reason I say this is because Thanos has the Infinity Gauntlet and if he finds all 6 Infinity Stones he could wipe out half of the Universe with a snap of his left hand. Thanos also plans to wipe out half of the universe in belief that it will solve problems with hunger. Thanos could even beat Superman with this power, so if DC merged with Marvel then making Thanos public enemy #1 would be top priority.
Potential DC allies of Thor would have Loki as an enemy since he has the capabilities of manipulation and orchestrating invasions on Earth. One DC ally of Thor he could be a potential enemy of is Wonder Woman which will be easy for her since she has a lasso of Truth to make him tell the truth. However he's a god so that could be a maybe. However he could also be an enemy of Batman because he's like the Joker, but more mischief than psycho.
10. The Mandarin
The Mandarin is a great enemy for potential DC allies of Iron Man because he is a prominent figure in terrorism since he is the leader of the terrorist organization known as the Ten Rings. One potential DC ally of Iron Man to have the Mandarin as an enemy would be Batman since he's a detective. However he could be a potential nemesis of Wonder Woman since he's got Ten Rings with magic powers.
Potential DC allies of Captain America would have HYDRA as enemies especially the fact that the organization had been recreated in the roots of a Government organization (S.H.E.I.L.D). A Scientist who worked for HYDRA during WWII had said "HYDRA was founded on the belief that humanity could not be trusted with its own freedom. What we did not realize is that if you try to take that freedom, they resist. The war taught us much. Humanity needed to surrender its freedom willingly. After the war, S.H.I.E.L.D. was founded, and I was recruited. The new HYDRA grew, a beautiful parasite, inside S.H.I.E.L.D. For 70 years, HYDRA has been secretly feeding crisis, reaping war. And when history did not cooperate, history was changed." Two potential DC allies of Captain America would consider HYDRA an enemy. Batman would make them an enemy since he believes they could possibly be linked to the criminal activity in Gotham City. Wonder Woman would make them enemy since they could have more of a history with the Gods old and New.
Potential DC allies of the Black Panther would have Killmonger as an enemy since he temporarily took over Wakanda to sell it's vibranium to anyone or just to get his revenge. He could be a tough enemy for Batman, if was an ally of Black Panther because if Batman finds out a Hitman like Killmonger sells Vibranium to potential enemies, then he might want to get involved. He would also be an enemy of Aquaman and Wonder Woman because they could be allies of Black Panther.
Potential DC allies of Spider-Man should have the Symbiote possessed Venom as an enemy since he could be trouble for them. One potential DC ally of Spider-Man who could have Venom as an enemy would be Batman because Venom deserves to be Arkham Asylum if he's this type of villain. He would also be an enemy of Superman because someone like Lex Luthor could use Venom as a henchman.
Potential DC allies of Miles Morales should have The Prowler as enemy since he is a dangerous vigilante that hunts and attacks people like a pro. In fact Potential DC enemies of Miles Morales should even hire him if they want to dispose of Miles if he does not find out that Miles is in fact his own nephew. He would be a potential enemy of Batman and could a tough one since he's killer combat fighting powers.
5. Winter Soldier
Potential DC allies of Captain America would also have the winter soldier as an enemy, especially Wonder Woman who fears he could be used by the forces of evil. Otherwise he should be good with being a Batman enemy since he's mentally brainwashed. Otherwise he's easy to un-brainwash and get him on the good side.
Potential DC allies of the Hulk would have the Abomination as a likely enemy of themselves too. One such ally could be Superman who can rival both abomination and the Hulk's strength. However Batman could also potentially be an enemy of Abomination since he has experience in taking people like Abomination down.
Potential DC allies of Wolverine could have Sabretooth as an enemy, especially the fact Potential DC enemies of Wolverine can employ his half-brother and rival to take him down. Batman would especially be a potential enemy to this mutant maniac since he's gonna think Sabretooth should be in Arkham Asylum.
Potential DC allies of Iron Man would also have Ivan Vanko (Whiplash) as an enemy since he wants revenge on Iron Man for the fact Iron Man's late father disgraced his dad a Russian Scientist. One Potential DC ally of Iron Man he could be an enemy of is Batman since Batman could consider him a formidable foe with the electric whips. He could also be an enemy of Superman since Lex Luthor could try to employ him like Justin Hammer and Cyborg since he can electrocute the Robotic Parts of Cyborg.
Potential DC Allies of Spider-Man would also have Mysterio as an enemy too since Mysterio attacked Prague and framed Spider-Man with illusions. He would be an enemy of Batman for the fact that he is insane as the Joker and has superpowers that create illusions. He could also be an enemy of Superman since he could be a threat that rivals Superman.