Now that we have seen “The Return” and we now know Rumple’s motivation for creating the curse in the first place and his sole mission since he has been in this world, we have a better idea of what he will ask Emma to do as her favor to him.
He has spent all of his time in FTL figuring out how to get to this world. Once he got here, he has spent all of his time trying to find Bae. It has been stated several times “Emma has a talent for finding people”.
I think Mr. Gold will enlist Emma to find Bae in some way shape or form. It will no doubt be in some round about way, just as he has done everything else. But I believe that is also why he says now that she is sheriff she just may be able to pay him back. She would have many more means at her disposal to track someone down either magically once she believes or through being in law enforcement.