but like what if madara did actually got pregnant with hashirama's baby?...how would tobirama react, how would fhe rest of senju and uchiha react? will the child have rinnegan and be the most op character in the entire naruto verse?

Anon, I honestly don't know...

Like, to begin with, we would have to think when and how this even happened. My "easiest" option would be making Madara trans, because there's no way they would have hormone therapy in their era😭 When that happened is tricky, because in canon they never were this close, but for the fanfic's sake let's just say it was after Konoha was founded (and iirc in canon Mito wasn't married to Hashirama yet)

I think Tobirama, like the rest of the Senju and the Uchiha, wouldn't be... accepting, because even if we consider there was no homophobia at the time (which I doubt), there's still the distrust against each other, but maybe they could accept taking it as a "guarantee" of the truce? That's how marriage kinda used to work anyway. But Tobirama wouldn't think like that, he would think of this child like a threat, like it's the child of two powerhouses, one of them being an Uchiha(!!), so he would want to keep an eye on them. If he would like his niece/nephew afterwards, I don't know, I guess it depends of which parent they would look like the most XD

About powers, I think the rinnegan needs the actual chakra, right? So this child wouldn't awake the rinnegan, even in an almost death situation, but they definitely would be op, maybe just not the strongest character of the verse, because both Madara and Hashirama don't have all their powers genetically determined, it also has the training component

Anyway, I hope my answer satisfied you, anon (wish I could know if you're a mutual or not)