One way the storylines could blend is through a magical event that transports Harry Potter characters to the DC universe where they meet Martians. Maybe a spell gone wrong by Hermione accidentally opens a portal. The Martians, with their unique abilities like telepathy, could interact with the wizards. For example, J'onn J'onzz could communicate with Dumbledore using his telepathy and they could team up against a common enemy like Voldemort who has somehow made his way into the DC universe as well.
Harry Potter and Superman would have an interesting interaction. Superman's super strength and invulnerability might be a source of wonder for Harry, while Superman could be intrigued by the magic Harry wields. Martians, like J'onn J'onzz, could interact with both. J'onn could use his shape - shifting ability to blend in at Hogwarts and learn about magic, and also team up with Superman to face any threats that cross over from the wizarding world.
Definitely not. Spider-Man is a prominent figure in the Marvel universe and has no place in DC Comics. DC and Marvel are separate comic book entities with their own distinct characters and storylines.
The Elongated Man is definitely a meta in DC Comics. His powers set him apart from regular people and make him a valuable member of the superhero community. He uses his stretching ability to solve crimes and protect the innocent.
Definitely not. Spider-Man is a Marvel icon and has no connection to DC Comics. The character's storylines and adventures are confined to the Marvel universe.