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A meta-commentary on Danny and Harry. Think of the parallels, the potential!
(Slaps car, "you can fit so much fanon into these bad boys!")
So without further ado, let's get started.
Green eyes: green like ectoplasm, green like the killing curse
Death didn't stick: Harry is the boy-who-lived. Danny is the boy-who-died.
Death-scars: Danny is often written as having Lichtenberg scarring from the accident that killed him. Harry is famous for his lightning bolt scar, received from the killing curse.
Power thrust upon them: Danny defeated Pariah Dark to save his town. Harry collected the hallows by accident/Dumbledore to defeat Voldie. Neither wanted their titles.
Eldritch entity: MOD and GK are portrayed in fandom as beings beyond time. They are not bound to one universe. Death is all-encompassing, it reaches ever corner of the multiverse that contains life. Thus, so do they.
Eternal teenagers: Depending on the depiction, there's a lot of takes where Danny and Harry stopped aging due to their titles/species.
Tired and timeless: AKA Overpowered and overworked. There's a lot of bullshit that goes on behind the metaphysical veil apparently. Being a god-like being isn't easy. There's paperwork and shit. Being immortal sucks.
Vacation AU: What do said immortals do when they need a break? Run away to a random universe for a new start. Hey, maybe here they can finally live a normal peaceful life, right? RIGHT? (It would be a pretty boring crossover if that happened, so sorry Danny/Harry. You're getting a love-interest at the very least.)
Antagonists obsessed with their moms: Vlad and Snape
(ok, that last one was added as a joke, but im not wrong)
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Just two Death Gods chillin' in your dimension: Danny and Harry are work colleges, just two dudes, two pals. Every century or so they pick a universe to dip in to live and relax a little. They're just a couple a eldritch tourists, no need for the locals to get so freaked out, honest.
Summons unclear: Maybe there's some mixup over which universes fall into who's jurisdiction. So when a cult tries to summon a death god they get two. And then Danny and Harry immediately start bickering and bitching. "No, I very clearly stated that this realm was/wasn't my problem!"
Alternates: They could be alternate versions of each other. Does something go wrong with the multiverse? The balance of life and death? Maybe they take each other's places or end up body-swapped. Or they could be reincarnated versions of each other and something unlocks memories of their previous life.
Godly matchmaking: Death and Time see their protégées suffering alone and set them up to support each other. Partners through the pain of eternal responsibility. Could just be platonic.
Soulmate AU: Matching death scars are morbidly romantic, right? Or it could be a write on your skin situation, where they write to each other and only meet once they've ascended. And it comes with the bittersweet knowledge that while they were destined to be together, but they were also destined for greatness born from suffering.