If he's actually her father (which he's definitely not but like I wouldn't past anything past these writers) I'm going to be pissed and not just because it's disregarding almost every decision that was made in the past four seasons, but because it breaks what is probably the most significant rule in the show- that Red never lies to Liz. Raymond Reddington is an extremely complex character who often has plans going on that are working beneath the surface. He continuously lies and deceives people making it virtuously impossible for the audience to tell what is real or not with one exception- the things he tells Liz are supposed to be sacred. If he truly is the father that means that none of the stories we hear from him are verified and literally everything we have learned up to his point could be a lie. Is Katarina really her mother? Is Liz really Masha Rostova? Is Katarina even a real person? Who the hell knows certainly not us. Making Red the father would give a character that already has an incredible amount of freedom no accountability. Trying to unravel any of the secrets of the show would become utterly useless because even if we are given an answer there's no guarantee that the answer isn't just a lie that the writers will go back on at any point.

Calling it now...