I saw this clever post on reddit once about wishing that when they become a witch, they retain their memories, appearance, and basically everything about themselves, but gain the abilities of a normal witch.
They also said you could work something out with magicas to trade death roll criminals/other humans for grief seeds or a help on a witch hunt.
But now with the existence of the mirror witch, we know that not all witches need to feed on humans, so it wouldn't have to be said way of obtaining energy.
The biggest issue with this wish is that it might be out of the limit of a normal human, eg. only someone with higher cosmic important may make this wish.