If she had been captured, what would have been Eva Braun's fate?

As far as I can tell from the literature available to me, nothing major at least when it came to what the Allies had planned.

The existence of Eva Braun as Hitler's girlfriend was not really known in the general public because Hitler felt that to fulfill his role as the Führer, it was better for him to appear single. By extension, the Allies were only dimly aware of her existence. When Walter C. Langer, a psychoanalyst working for the OSS, wrote his profile of Hitler's sexuality 1943, he neglected to mention Braun or had no knowledge of her at all. It also doesn't appear that the Soviets knew of her.

Also, aside from Ilse Koch, wife of Bichenwald Concentration Camp commander Karl-Otto Koch, the wives of high ranking Nazis were not tried, except in the denazification process, where they were usually classed as Nazis and thus stripped of some of their belongings (if they were the proceeds of Nazi activity) and of some of their rights (i.e. not being allowed to vote in elections until 49). Braun was posthumously classified as a Nazi by a Munich denazification court and thus stripped of all assets she had received from Hitler, which were virtually all her belongings.

It is impossible to say with absolute certainty but give these circumstances, the most likely scenario is that she wouldn't have been treated much differently from Himmler's wife, Margarete, or his mistress, Hedwig Potthast, both of whom were questioned by the Allies and declared Nazis or fellow travelers during their denazification and thus faced some ramifications though nothing overwhelming.

As far as we can tell, the Allies did not have a plan for such a scenario such as the capture of Braun.

Sources:

Ian Kershaw: Hitler.
Heike B. Görtemaker: Eva Braun. Leben mit Hitler.
Anna M Sigmund: Die Frauen der Nazis. (caution with this one. It's very sensationalist).