I’m not sure the doll is Bae’s (or that it’s a doll OF Bae). In the clip doesn’t Felix say something like “the things we haven’t thought about in years can still make us cry”? That implies that the doll is related to something that happened long ago (that we might not know about yet) and that hasn’t been thought about in a long time. Rumple has thought about Bae and all things related to Bae probably on a daily basis for the past 300 years. The doll is supposed to represent something from long ago that Rumple has pushed from his mind (for whatever reason). The same would go for Belle, I know a theory is that the doll is related to her as well, but Bae and Belle are people who have always been on his mind. I think the doll represents a person and/or event that happened that Rumple has repressed/forgotten.
Plus, I think we all feel like we know who Rumple is by now so it would be cool if there’s yet another chapter of his life we don’t know about (the man IS 300+ years old lol). Idk if he’s been a lost boy though cause we already know he was raised by spinsters.