1. Cozy Glow: The reason Cozy Glow is behind the missing party supplies because in season 10 in the comics Chrysalis breaks free which means the others must have break free one by one, and Cozy might be the last one, and gotten mad because her friends Tirek and Chrysalis gotten freed and have to leave her behind. Feeling betrayed Cozy Glow decided if she can't have friends one can. An older Cozy will tried to ruin Maretime bay day. She has help from someone who thinks she or he is her friend.
2. Sprout cloverleaf: I don't see Sprout as a villain in the movie but I know have he'll play a bigger role in the game. Feeling jealous of Sunny's work he decide to frame her by stealing all the decorations to make her get the blame. Lucky her friends defend her. At his final boss fight he use Sprouticus maximus 2.0.
3. A new character: I think a new character is a good idea, and they might be reformed. Because in the 2017 movie we got Tempest Shadow who work for the storm king, and then she got reformed.