I'll continue with my theories.
Stage 4 boss: Could be like Kasumi Shindou from Marine Benefit's Stage 4 (each character takes different spellcards), a returning character or a new one.
Stage 5 boss: Related to Stage 6 boss, probably you'll fight her because she wants to protect the final boss.
Stage 6 boss: Here's the interesting stuff. There are probably two Stage 6 bosses, like in Imperishable Night, but you don't choose which one you'll fight. If you're carrying the corrupted weapon, you'll fight an important boss, but not the one who started the whole incident (like Eirin). If you're not carrying any weapons, you'll fight the true boss (probably a Mirror Youkai since it's said that DDC is related to mirrors).
Extra Boss: Extra Midboss could be Kagerou on steroids or a returning character. Extra Boss is someone new (obviously), related to the story, who could have some kind of relationship to Stage 6 boss/bosses.