Given the lack of knowledge about this 'ftm cock fusion tg story', it's difficult to precisely explain the key elements. If we break it down, in an FTM transgender story, elements such as body dysphoria, the search for medical or social transition support, and the building of a new masculine identity could be important. For the TG aspect, it expands to the broader transgender community's experiences like discrimination and acceptance. But 'cock fusion' is not a term that has an obvious meaning in this context, so it's impossible to accurately define it as a key element without more information.
As I said before, without more information, it's hard to pick out the key elements. But generally, in an FTM context, key elements could involve the transition process, identity discovery. However, 'cock fusion' is not a common term so it's hard to say how it fits into any key elements.