They are entirely different in meaning, though there will be some contexts in which either is possible.
"To" with a verb of motion ("Come back") expresses the destination or target. "I'll be coming back to you" is a simple statement of intention, with no necessary implications.
"For" is most often a benefactive, so a possible meaning of "I'll be coming back for you" would be "I'll be coming back for your benefit" (though exactly what that benefit would be is not expressed). It does not even say that I'll come back to you: for example, if you have been accused of some crime, but I am guilty of it, I might mean that I'm coming back to give myself up, and get you exonerated - you might not even see me.
But actually, that interpretation is less likely than another meaning of "for" which is "for the purpose of". Again the precise interpretation is context dependent: if the object is an event, it will usually mean to participate in the event (I'm coming back for lunch/for Christmas/for the lecture). But with a personal pronoun, it will be interpreted as "to get you, or take you somewhere".
I'm coming back to you. = I'm coming back, and I will come to where you are.
I'm coming back for you = (most likely) I'm coming back in order to get you and take you somewhere.