I have a few theories. Rumple’s probably still making up his mind about what favor he wants Emma to do for him.
First, the favor that Emma owes Mr. Gold may be for her to retrieve Belle back for him. Although Mr. Gold may not yet know about Belle being alive in SB, by the time he learns about Belle, Mr. Gold may call upon Emma to get her back. We already know that we'll see Belle again in the season finale.
The next theory I have is a tad spoiler-ish. Based on the episode summaries for 20 and 21, Emma is going to have to decide weather or not to stay in SB. Perhaps Mr. Gold's favor will be for Emma to remain in SB until the curse is finally broken.
Finally, I like the idea that Emma might be called on to discover August’s true identity and report back to Mr. Gold. Episode 19's summary description has Mr. Gold trying to uncover who August really is. Maybe he'll get Emma to help him since she has become pals with August.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy