A spin-off featuring Vi, Caitlyn, Ekko, and Jinx is almost certain, given the popularity of the characters and the loose ends that need to be tied up. The producer has already confirmed that we will see them again, but he didn't say when or where. They will probably do with Arcane what was done with The Walking Dead, separating the characters into different spin-offs to continue the arcs and increase the visibility of the series' universe. I'm quite optimistic about that.