Back when the 1st season of Symphogear ended and the 2nd season was announced, there were some staff changes. Perhaps the most notable departure aside from character designer Satoshi Koike, was the director of season 1: Tatsufumi Ito. Both of them departed to work on other projects and this left some holes to fill on the team. So the people in charge took a look at those who remained behind. A storyboard artist and director who worked on a few episodes of season 1, Katsumi Ono, ended up being the one to fill the hole and was announced as the director of season 2 and later on, season 3. Katsumi Ono made his directorial debut in 2006 with the anime, Hataraki Man, though he's probably best known for his work on the 3rd Yu-Gi-Oh series: 5Ds. This is where everything begins.
During the production of Symphogear G, Katsumi Ono made a vague tweet about how he had "drawn that" again. It wasn't until months later that people connected this to a scene from episode 1 of G when they realized that Hibiki's punch attack was modeled directly after Junk Warrior's signature "Scrap Fist" attack from 5Ds.
For awhile, this was the extent of the connection between the two series.
However, when Symphogear G ended Katsumi Ono was announced to be the director of the 5th Yu-Gi-Oh series: Arc-V. There was some speculation that he would not work on the next season of Symphogear as a result, but he somehow managed to fit it into his schedule because when GX was announced he remained as the director. So now that he had successfully worked in a reference to Yu-Gi-Oh in Symphogear, would the same be true in reverse?
Sure enough, when Arc-V made it's debut on TV there was a key scene in episode 2 when the protagonist, Yuuya, utilizes a new form of monster summoning: the Pendlum Summon. The "energy" given off by this summon was detected by the city's duel network, which led to a very familiar looking scene for us Symphogear fans.
We had come full circle, Katsumi Ono's work on 5Ds had made its way into Symphogear and now his work on Symphogear had made its way into Arc-V. And again for awhile, that was all there was. But Yu-Gi-Oh is a lengthy series that often goes on for well over 100 episodes, so what happened when he was working on both Arc-V and GX simultaneously? Well, the results didn't make themselves clear until more recently.
In episode 75 of Arc-V there is an intense duel featured between two characters. At the climax of the duel, one character summons a monster: Satellite Cannon Falcon. True to its name, when it came time for the monster to attack it flew into space to charge its cannon and unleash devastation. However the scene looked incredibly familiar to one from the first season of Symphogear.
While perhaps both of these can originally be traced back to the Satellite Cannon of Gundam X fame, there is no denying how similar the scene is framed as it plays out. Of course, it doesn't end there. Just 2 episodes later, fans noticed yet another remarkable similarity.
In episode 77 of Arc-V, Yuuya's best female friend and clear love interest, Yuzu, is shown struggling through an intense duel of her own. Despite being on the verge of winning, she loses at the last second and unfortunately for her this was no normal duel. Arc-V also features 5Ds signature "riding duel" or better known to the internet as "card games on motorcycles." As a result of her loss, Yuzu's bike is thrown off the track and crashes into a nearby building which causes a huge explosion a few seconds later. Watching from afar hopelessly the scene zooms in a shocked Yuuya before it freezes dramatically as he calls out Yuzu's name, thus ending the episode. Now if you think back to episode 8 of Symphogear G this might sound somewhat familiar: Hibiki and Miku are visiting a tower when they come under attack by the Noise. Despite Miku grabbing her hand, Hibiki ends up falling from the tower and transforming into her gear. She immediately prepares to go back to save Miku but to her horror she looks up to see an explosion engulf the tower. The scene zooms in on her shocked face before it freezes dramatically as she calls out Miku's name...
Finally in episode 79 of Arc-V, Yuuya summons his newest monster: Enlightenment Paladin. One notable characteristic of this monster is that it dual wields two swords. At the climax of the duel Yuuya has Enlightenment Paladin attack his opponent, which results in Enlightenment Paladin taking its swords and combining them into one long weapon, which it then twirls around before dramatically slashing the opponent and sending them flying into the sky. Again, for Symphogear fans this might sound like someone we know. If you take a look at episode 1 of GX you can see Tsubasa perform her signature "Wind Wheel Flame Slash" which is essentially the same move: two swords combined into 1, twirl and then slash.
As of this post, Arc-V has aired 81 episodes and doesn't show signs of ending anytime soon. So just how deep will the Symphogear/Yugioh rabbit hole go? Can we expect anything from a potential 4th season of Symphogear? Will it be titled Symphogear 5Gs? Will Tsubasa learn to play card games on her motorcycle? Only time will tell.