The phrases "my future wife" and "my wife-to-be" are effectively synonymous and interchangeable. Both refer to a person who is not yet your wife but who will become your wife someday. The phrase "my future wife-to-be" is therefore duplicative and overdefined.
In a comment that originally appeared beneath the posted question but was subsequently moved to chat, Stuart F offers the following assessment:
"Future wife-to-be" can be non-redundant. If you're single but thinking about how you might plan your wedding if hypothetically you met someone, then you would be thinking about your future wife-to-be, because she's not yet your fiancee or wife-to-be.
This is an interesting argument, but I don't find it entirely persuasive. My objection to it is that one might argue just as plausibly as follows:
If you're single but thinking about how you might plan your wedding if hypothetically you met someone, then you would be thinking about your wife-to-be, because even though she's not yet your real-world fiancee, she is your imagined wife-to-be.
I don't think that inserting "future" before "wife-to-be" enables the speaker to make any logical progress toward meaningfully distinguishing between an imagined or not-yet-identified wife-to-be and an actual or known wife-to-be.
In casting about for an example in which "future wife-to-be" might function nonredundantly in all its parts, you might argue that "my future wife-to-be" could be useful in a context where you wanted to distinguish between a person whose status as your future wife is farther in the future than the person you might designate as "my current wife-to-be"—that is, the person who is nearest in line to become your wife.
But the proposed distinction here seems poorly conceived and somewhat caviling, given that both of the people thus designated are already (in a strictly logical sense) "current wives-to-be." A clearer way to indicate their expected order of appearance as actual wives might be something along the lines of "the next person who will become my wife" and "the next-plus-one [or next-plus-two or whatever] person who will become my wife." I doubt, however, that occasions for making such a distinction arise very often.