In the US, I've seen the term would be X referring to something that would have, under given circumstances, been something, but for some reason, it didn't come to be this way. For example, a would be doctor might refer to someone who was studying and was ready to receive their doctorate, but then suffered an accident that caused this person to miss their final exam, thus (in this case, temporarily) denying them this status.
In that sense, I don't believe that in the case of certain future marriage, you can use this idiomatically to mean husband/wife to be. But as far as your second question, the term X to be can be applied to anything that will become that thing.
Here's a short, quick list of examples using X to be:
Graduate to be
Someone who will soon graduate from a university
Husband to be
A man who will soon be married
Lawyer to be
Someone who, presumably, will become a lawyer in the near future
...and so on.
Keep in mind that here, using X to be, you are referring to a title, status, or other such stature. Adjectives don't work there. It is unnatural to say happy to be, for example, with this meaning.