Doing a thought project of what this crossover worldbuilding would be like if DC, Marvel, Hellboy, and TMNT all existed in the same setting.
-Hellboy at some point joining Justice League Dark. Him and Constantine notably not getting along, bickering and trading insults at one another.
-League of Assassins, The Hand, and The Foot having some form of history with one another. Perhaps both The Hand and The Foot being distinct but otherwise related organization branches connected to one another.
-With DC involved perhaps metahumans/mutants (Both could be a fusion of both in this setting) might be more accepted by society, meaning the turtles and other mutagen mutants like Splinter, Leatherhead, and such would not have to stay hidden in the shadows. The same with Hellboy being kept a secret by the government.
-Can only imagine the conflict Lobo would also have with the Guardians of the Galaxy. Both are bound to run into each other every now and then.
-Thanos having some form of respect towards Darkseid. To that point when supposedly the Infinity arc in Marvel occurs, it's the only planet he spares of wiping half of life from.