I'm pretty sure this is instrumental case, as accusative would be "manželku."
To test my little theory, I did a Google translation for "you will be a good wife," and it gave me "budete dobrou ženou," which is instrumental.
I'm sure someone with more knowledge will be able to explain WHY it is the instrumental, because I can't help with that!
"To be someone/something" can be translated both with nominative and with instrumental. There are certain rules when one or the other is better. I think they were shown by me or someone else in some other thread. This text in Czech is about that https://cs.wikipedia.org/wiki/Nominativ#Konkurence_nominativu_a_instrument%C3%A1lu