Very curious to see how they'll explain Yuto's presence in Duel Links since every anime is post-script. The Declan event has him worrying about Yuya's vitals as the boy duels so is Duel Links' digitisation of the consciousness going to start splitting apart the Yu-Boys or something?
Kinda funny he's worried about Yuya but completely ignored Yuzu who not only has a similar condition, but also, her original identity splitting the worlds is pretty much the entire reason the main plot of Arc-V series happened in the first place
There's a couple of different ways they could bring back Zarc, the easiest would be to have him be an "echo" like the Dark Signers or SK Jaden, in which case they would be able to recycle the Devil-Yuya design from previous video games like Duel Links loves to do. Personally I would prefer Zarc in his original human form before he became the Supreme King.
Probably the same way Kalin and Dark Signer Kalin can exist at the same time. Also, some of the characters in the game are dead anyway, yet appear in this game anyway.
I think that Duel Links itself is traveling through time and universes and creating copies of the minds of Legendary Duelists. It's the reason we have two or three versions of a bunch of Duelists, and why some Duelists who died are now in the game.
I think the Dark signers, bruno or supreme king. Characters like them are replicas created from the Memories of the Protagonists. Something familiar from KH chain of memories Riku Replica.