If (and this is a big if, mind you) they were to show additoinal backstory for Regina and Daniel, I would highly doubt it would be as simple as he's the stable boy and her horses were at the stables. The writers would find some way to twist things up a bit.
I could see a twist on this like having Rumple be the person that "found" the stable boy for Cora (who was looking to replace a previous Stable Boy that sassed her one too many times), in order to take a Regina that is very much NOT into dark magic, and giving her an extra push in that direction. Not pre-arranged, mind you. Cora knows nothing of this little bit of subterfuge. Rumple is just playing on his fortelling of the relationship between Danial and Regina, and expectations that Cora would do exactly what she did once she found out. Because, as we know, he needs Regina to be the one to cast the curse. Of course, the twist that Snow was the one that told Cora was just icing on the cake (and maybe not expected by Rumple any more than by anyone else). And if you REALLY want to twist it up, make it so that David is almost selected as the Stable boy, but narrowly misses that fate when Cora chooses Daniel instead.
Now, having presented a OUaT type twist on the story, let me go on record saying that I don't need to see this particular backstory. Unless it has extreme relevence to the story, Daniel and Regina's background should just remain as a mystery, and let everyone decide on their own how they think the backstory worked out prior to what we saw in the Stable Boy.