I don’t think his favor will be a power play but I do think he has enough leverage & say so to make sure that Emma upholds her end of the deal. After all, he goes to great lengths to explain to her that rules are rather like deals & they’re what make things run smoothly in this world. So that leads me to believe that if Emma did choose to ignore her ‘debt’ to Gold, then it would throw things off balance & perhaps lead to dire repercussions, not only for Emma but for those she cares about. But as for what he’ll ask for, I truly believe that after the events of ‘Hat Trick’, he’ll ask her to side with him, and use her magic ( that she may or may not have – I think she has magic ) to help him meet his goals; whatever they may be.