If there's a Phantasm, it all makes sense. 'Cause, you know, 7 x 2 = 14
By that logic, Marisa confirmed for a stage 4 boss of Touhou 16. Because 2, 4, 8, 16, obviously.
Anyway, I wouldn't be too surprised by there being another theme for the final spell. It'd fit with DDC's PCB parallels (shot types are Reimu/Marisa/Sakuya A/B, Cirno as stage 1 midboss), and it's not like PCB is the only official Touhou game with a final spell theme (it's in 1, 7, 8, 10.5, 12.3 apparently). Maybe I'm just expecting one after hearing one in Mystical Power Plant, though.