He actually didn't go into the war with a wish in mind, because I don't think he felt like he wanted anything more than what he already had aside from the extra glory that would come from participating in the war. He kind of just... wanted to add the war to his resume so I think his plan was to just stick the Holy Grail on his mantlepiece so he could brag about it at parties or something... If he absolutely had to wish for something, it would probably be something kind of minor.

Like cake, maybe.