Girl!Dexter. Her gender is just another reason Harry adopts her, but despite the statistics she still becomes a serial killer.

Dexter Peggy Sue: Dexter wakes up back as a child/teenager (before Harry died), with all of his experience with life and killing. Not to mention years of seeing and getting advice from hallucination!Harry, which doubtless does not match up entirely with what real!Harry would say. And experience pretending to be a normal person. And, you know, the fact that killing animals won't cut it anymore.

So, one day God says to Joan, "I need you to change your name/get a really good fake ID and go to Miami." The name on her ID is Lumen Pierce.

I've never seen Joan of Arcadia, but I'm pretty sure that she was a nice and normal girl before god started talking to her. What if she wasn't? What if she was more like Dexter, only faking being normal, and holding off on her darker impulses as long as she could because if she could love anybody, she'd love her family?

Hey, you know what would be a fun crossover? Dexter/Journeyman. Several possibilities: Dexter travels through time (and let's just say he solves problems a bit differently than Dan and Olivia do). Deb travels through time, and finds out about Dexter. Dan has to help Dexter, and may or may not find out about him.

Dexter Morgan, not a serial killer.  Nothing out of the ordinary about him.  But then one day, he comes home and finds a dismembered Barbie doll in his freezer, connects it to the Ice Truck Killer, and is all D: about it.