I've noticed that Indians use the term would-be in place of fiancé/fiancée. Usages like "Meet my would-be" and "This is my would-be" are common in introductions. I used to wonder if this is just an Indian usage or if we got it from the British. I have never heard it being used in American movies or on TV. (I also used to wonder if people used "He/she is my would-have-been" if the wedding was called off after the engagement. :-))
In England we use would-be as a pre-modifying adjective. Used as such, it means something that could have happened if conditions had been different. For example:
Jim claims that he designed the iPod but that Steve Jobs beat him to the patent office. These days, the would-be millionaire takes his 40 minute dinner break at the local MacDonalds.
Here we can see that Jim could have been a millionaire had he submitted his design sooner. As it is though, he missed out and lives a normal life.
With that in mind, we could say would-be husband/would-be wife but the meaning would shift. It would mean somebody that could have been your wife under different circumstances (perhaps they were all set to marry when the guy got drafted) but are not. It certainly is not the same as fiance.
Hope this helps.
As a point of interest, if you wanted to adapt the phrase to the meaning, grammatically it would have to be will-be wife, but that doesn't exist.