Maybe Orochimaru. Despite being a villain in the past, he is a master of forbidden jutsus and experiments. He could see Naruto as an interesting subject and give him a new arm with some dark powers, which Naruto would then have to learn to control for the greater good.
There might be such fanfics in the vast world of fanfiction, but it goes against the original canon of the 'Fullmetal Alchemist' series as male pregnancy is not a part of the normal story. However, some fanfic authors like to explore very unconventional and out - of - the - box ideas.
I recommend you to read the two novels,'Sweet Oxide', and' There's a Dragon on Him.' " Sweet oxygen " was a novel about a tattoo artist. The author was Zhang Yuqing. On the other hand," There's a Dragon on Him " was a novel with tattoos as its theme. The male protagonist was a retired soldier with a real dragon tattoo. These two novels both involve the male protagonist having a tattoo. I hope you will like them.
According to the information provided, it was impossible to determine the exact plot of the " stuffed half an arm " in the manga for no reason. I suggest you check out the relevant comic reading platforms, such as the comic market, reading comics, etc., to obtain more accurate information.
While waiting for the TV series, you can also click on the link below to read the original work of " Little Fox Demon Matchmaker " to understand the plot in advance!
In the manga, Sasuke doesn't regain his Rinnegan. The story progresses without this particular change. The power he had before is maintained without reverting or regaining additional abilities related to the Rinnegan.