Rumple wanted Ella's first born probably because of the same reason he wanted Snow's. Ella's and Thomas's baby would probably be born from true love so he/she can break the first curse like Emma did but I guess he had to settle with Snow's baby after he was imprisoned by Ella.
I know there is no way that Rumple had planned it, but in the end it turned out to be a brilliant move. Emma asked him to give up any claims for the baby, and in exchange for that she would owe him a favor, which he later used to find his son. Maybe it's just a coincidence, maybe he predicted it from advance due to his ability to see the future... who knows...
Honestly when I first started watching Once I thought Rumple was a peado or something (why else would someone want babies for a deal) but I remembered from Season 3 that Regina said that (im paraphrasing) "the amount of spells involving baby parts will surprise you" to snow when zelena was after her unborn child so maybe the baby was needed for a spell.
Guys, we're overthinking things. (So, I'm going to overthink it some more. :) )
Rumplestiltskin was all about getting babies, and no one knows why. That was the original fairy tale.
Once's version of Rumple actually only goes after 2 babies. 1 was Alexandra, the other was the deal he made with Cora (which came straight from the Rumplestiltskin fairy tale). Although he was interested in Emma (Very Interested, in fact), he never actually tries to make a deal for her, just her name.
Throughout the series, Rumple is a deal maker and master manipulator, and he does not make ANY deal unless it has some significant benefit to him. He traded for a few strands of Snows Hair, David's cloak. and other seemingly random items. All as a part of some master plan, which was to get his Son back. This is speculation , but I think He knew he would need a 'guide' in the LWM once the curse was broken, and what better way than to have a bargaining chip to exchange for the favor.
In The Shepherd we learn that Rumpel took James so he could make a deal with King George. Maybe he runs some sort of adoption agency so people will owe him favors. He does refer to babies as merchandise. This reason is slightly more legitamit then in the traditional story.
I notice one conflict with the assumption above. In The Outsider, Rumpel is about to leave to find his son by himself. After Belle is shot, his plans are delayed and he later decides to take Emma with him. This means he wasn't planning on having Emma help him find his son, so his deal with Cinerella would have been unnecessary. He may still have wanted to have leverage on Emma for other reasons.
By the time of Ella's wedding Regina already announced the curse, so Rumple probably knew that Ella's baby would not be born in time. He must have been planning ahead, so he would have leverage over Emma or Ashley 28 years later. The leverage could not have been specifically to help him find his son, as that would contradict earlier events of season 2.
That was the deal in the original fairytale, which was more or less illustrated in the season 2 episode 'The Miller's Daughter'
The Cinderella one was more or less between the two fairytales (coz Rumple absolutely has to meddle with everything) the deal and Cinderella eventually fooling Rumple was a reference from the "Rumplestiltskin" fairytale.
As far as I know it wasn't exactly clarified why he wanted it. But that's just one in a long list of deals, honestly, I didn't understand the motive behind most of his season 1 deals, except for stocking his shop.
Begfhn wrote: In The Shepherd we learn that Rumpel took James so he could make a deal with King George. Maybe he runs some sort of adoption agency so people will owe him favors. He does refer to babies as merchandise.
Lol this one's a good explanation. Make a deal with one person, take their child, then make a deal with another person and give them the child.