I think what would end is this:
After defeating ZONE, the signers have their lives in peace. They get autographs from the people for saving the planet, including Sly who pretends to be signing Yusei's Stardust Dragon and lying to him that he'll give it back later.
Yusei and the signers suddenly got a call from the Crimson Dragon telling them that its time to return all 6 of the dragons to him. Just when they were about to leave, Yusei wonders what happen to his Stardust Dragon and when Sly was gonna return his card. However, he and the others soon learn from the kids that Sly actually stole his dragon.
Yusei and co. search for Sly and chase after him. Yusei demands it back, but Sly refuses. And if he wants it back, he'll have to duel him for it (maybe Sayer has a psychic deck since he and Sayer's hair are alike). Sly summons the Stardust Dragon for the first time and demands that he attack Yusei but.... Stardust Dragon ignore's Sly's calling and actually attacks Sly since he isn't a Signer. Yusei calls SD Dragon to return to his deck.
With Sly's defeat, he shows tears in his eyes. It turns out Sly never made any real friends and wanted the card so bad, he wants power and defeat everyone that stands in his way. Especially being hurt when his parents were divorced and never had time for him. Yusei and the gang forgive Sly and it was time for all the dragons to return to the Crimson Dragon.
After that, they move on... but never forgotten their bond of friendship.