@Twilight Sparkle’s #1 Fan  
It’s a good argument, and I have to agree with the reasoning. I only went with Phosphorous because I don’t think he’s ever shown up in any Batman adaptations. Which given his powers and looks seems odd.

@Ardashir  
I personally think she’d work better as the Joker due to the much more personal and historic relationship the two have with their respective opponents. They know their rivals better than most people do, if not the most out of everyone else.
 
The Joker’s brightly colored outfit and stark white skin also highly contrast with Batman’s pitch black outfit just as much their clashing demeanors do. Joker’s high-pitched hamminess vs. Batman’s deep-voiced stoicism. This plays well with Daybreaker’s and Luna’s differing personalities too. Daybreaker is blazing (literally) with light and has a sadistic theatrical demeanor in comparison to Luna’s darker coloration and stern-faced heroism. Not to mention Joker & Daybreaker have huge egos that differ against Batman & Luna’s selfless nobilities.
 
I also think Daybreaker fits the mold of the Joker’s “One Bad Day” worldview rather well. Daybreaker represents the dark possibility of what would happen should Celestia ever let go of her morals and just snapped one day. If she had too much pressure upon her and couldn’t deal with it under any saner circumstances. Daybreaker is quite obviously insane with her maniacal laughter and sharp-toothed slasher smiles. She’s what would happen if “Celestia ever had just that One Bad Day.”
 
That’s my two cents, at least.

@Eroraf86
 
@Twilight Sparkle’s #1 Fan  
Glad everyone likes it, but I wonder – would Daybreaker work better as the Joker or as Dr. Phosphorous?

@Ardashir  
GIVE THIS TO ME! NOW. This is legit perfection!

HOLY FUCK, ANDY! You are a god!

@Ardashir  
FANFIC. NOW. THERE IS SO MUCH GOOD MATERIAL HERE.

@Eroraf86  

 
“You’re dressed all in black? I get it, you’re sad. You need to lighten up, Batsy!”

Holy shit, that is awesome! I demand to see a fanfic based on this!
 
@Ardashir  

 
Now THAT is an awesome mental image.

 
We are Vengeance. We are the Night! WE ARE BATMARE!

If Luna is Batsy, then what Bat-Rogue does Daybreaker come off as? She’s no Penguin or Riddler, that’s for sure. Maybe she’s the Joker?
 
… The Joker, with the power of a literal fire-spewing star at their command. Yeah, we’re all dead.

WE ARE THE NIGHT!