Serenes Forest has a thread about Fates trivia and it sources it from TV Tropes which has no source:
https://serenesforest.net/forums/index.php?/topic/61734-interesting-unused-content-trivia-in-fates/. That said, some of the users I think dug into the game's files. For example, Takumi might have originally been called Takuma who reminds me of Mr. Karate considering his abilities in Heroes and notoriety in both Fates and Heroes.
I don't know how his fireball works, but it's like this invisible palm strike. I have no idea what the hell you're supposed to do against that other than pray you were holding back to block it when he does that move. Ditto on Benimaru's lightning strike from the sky that starts above you.
Anyway, GameFAQs also has threads about this:
https://www.gamefaqs.com/boards/114533-fire-emblem-fates-conquest/72973366, one of which links to someone who translated the Making of Fire Emblem 25th Anniversary Book on Tumblr:
http://jokerthebutler.tumblr.com/post/134931464083/and-more-interesting-bit-of-fates-plot-and-the. Reminds me of reading French interviews on Izumi Tsubaki, the creator of Gekkan Shoujo Nozaki-kun, and someone translating. I can read French well enough, but I digress. So, yeah, assuming the translation is correct, then the original draft by Shin Kibayashi for Fates did not have Hinoka at all. I don't think she was even a random unit or a retainer. She just did not exist plot-wise.
What's interesting is that Hoshido was supposed to have only 3 siblings. I'm guessing it was Ryoma and Takumi who I remember were supposed to be based on lions -- Ryoma's lobster mane -- and probably Sakura. As the draft was altered -- let's say before Hinoka was created --, the developers, Intelligent Systems, might have thought it was unfair that Hoshido had 3 siblings while Nohr had 4. Kagero was probably added by request and by Kibayashi who is said to be responsible for the all the siblings, their characters and roles, at that point.
Hinoka's trivia section on her entry on the Fire Emblem wikia states that according to the Making of Fire Emblem 25th Anniversary Book, the artist, Yusuke Kozaki, drew her first before Camilla. I have no idea if he was just drafting character designs and he drew Hinoka for some reason before Nohr sister Camilla, however, Hinoka did inspire Camilla's design to be the opposite where Hinoka was a tomboy while Camilla was more "feminine" and "plot-heavy".
Anyway, right below it, it's stated that Hinoka as a character and as the Hoshido older sister never existed in Kibayashi's original draft for Fates. Page:
http://fireemblem.wikia.com/wiki/Hinoka#Trivia.
Neither of Kagero's entries -- wiki and wikia -- list anything about her originally being the Hoshido older sister and I don't know -- haven't checked -- if either Fire Emblem wikis have a beta section. I'm pretty sure one of the Zelda wikis and Final Fantasy wikis have entries about beta or early development of some of the games. Damn sure Metal Gear has one at least for Metal Gear Rising: Revengeance when it was originally Metal Gear Rising.
TV Tropes and the Serenes Forest thread which cited TV Tropes stated that Kozaki liked Hinoka's design and requested she not only be put into the game, but as a Hoshido sibling who at that point, might have been Kagero. The weird thing is that in the thread, it's mentioned that Kagero looks suspiciously like Queen Mikoto. The siblings in general have weird hair color, except for Nohr where it's only Camilla and I think that's explained by Garon having multiple wives, but Sumeragi, before meeting Corrin's mom, only has one wife, right? Then why do his kids have hair colors ranging from dark brunette, dirty blond, red, and pink? Was his wife a rainbow or something? It's mentioned that Takumi was supposed to be a redhead like Sakura and later, Hinoka, so maybe his first wife was a redhead. Even without Kagero or Hinoka, that would make sense. Ryoma takes after his dad and Sakura and Takumi take after their mom. With Kagero, then father's hair while Hinoka would be mother's. Add in Corrin's hair supposed to be black by default, Kagero supposedly being the original Hoshido older sister, and Kagero looking like Mikoto and I have this weird idea that Kagero could have been Corrin's actual blood-related sister.