Danny is a progenitor vampire.
This mostly comes from how I've been seeing a lot of vampire media (which is a surprise in May but quite welcome!)
It got me thinking, modern interpretations of vampires have a lot of powers that are basically just watered-down versions of what ghosts have in the show. Plus Danny is more than powerful enough and I'm sure Vlad (masters not the impaler) would be so mad that Danny stole his shtick.
(flight, shape shifting, invisibility, super strength/speed, a weird extra move here and there's pretty common as well, it would also explain why most vampires only want blood since that's valuable life essence a ghost could easily use.)
So what if, while being sent on a time mission by Clockwork; 14-18 year old Danny comes upon a situation where he needs to extend his power to a human. (This can be any character, like dracula or some other elder vamp from other media)
After this Danny assumes that any lingering effects would flush out of their mortal buddy's system like a possession would. Since he finished up with his mission he goes back to Amity park. He comes back and everything is normal, the entire town is still the mildly ecto-contaminated metropolis he knew. However, when he catches a glimpse of the news, he and his friends see that vampires have been raging all over! Danny is shocked at what he sees! He suits up and gets ready to fly--is that his mortal buddy he was just telling Sam and Tucker about on screen?

















