Probably to do well in her contests, everyone to be happy, do well, something simple like that.
In my opinion, though, May could have wished for Ash to be together with Misty. May did give hints in the Volbeat/Illumise episode, and seemed to recognize Ash's relationship with Misty. Again, this is just my opinion. I really didn't watch the movie fully so I don't know when the movie could have taken place.
Why would May waste a wish on something like that? I mean if we have to get literal here...
The only way I could see her doing that, is if she liked Ash, and she knew she was never going to be with him and she wanted him happy because she thought he liked Misty.
But if she liked Ash, then she could have just wished for him to like her anyway, since then he WOULD be happy with her because... he would like her.
But, she could have wanted him to have free will and make the decision himself. But if she DID want him to have free will, she wouldn't want to be wishing for him to like somebody, because then he wouldn't have a choice in it.
UNLESS he already liked Misty (which I doubt), then she could wish for it, because then it wouldn't matter about decisions...
But if the Ash and Misty BOTH liked each other in the first place (again, doubts), then May wouldn't need to wish for them to like each other seeing as nature will probably take it's course, and she could help them with that WITHOUT having to waste a wish.
Unless she was obsessed with their relationship in a stalker kind of way (which is the only way I see her wishing on something like that), but if she was, then she would have shown more hints when Misty came back. None of them even seemed to hint at the Pokeshipping plot at all, which seems to me that Misty doesn't like Ash anymore, Ash never liked her in the first place, and that May didn't need to get involved, so that entire wish would have been pointless in to begin with.
May is nice and all, but really.... Seriously, why would May use a wish on just THEIR future?
But hey, that's just what I think... And I'm not trying to attack anyone, I'm just saying...