(SPOILER WARNING)
Well, we just found out yesterday that there will be a season 4 and I am so excited. I have been thinking about what might happen and I love making theories. ^^ This is what I think will happen. The episode will either start off either Marco being shocked that Star left or it will start at Star being at Mewni. I think that Marco will start to think about his feelings towards Star, but Jackie will be really supportive and she won't hate Star for having a crush on him. I don't really see her like that. Maybe Marco will go in her room (or what used to be her room) and the memories he had with her will flash in his head. If this doesn't happen, maybe it will start off with Star arguing with her mom. Maybe Star would want to go with her mom to fight Toffee, but she doesn't think she is ready yet. Star is sad that she might have destroyed her friendship with Marco and she thinks that she probably hates her. She did't want Toffee to take Marco away again like last time so instead of spending the summer with him she decided to leave him to protect him. If Toffee's finger is still in Star's old room, then Toffee and Marco might meet again and he might try to bring Marco into the dark side. And this isn't my theory this is someone else's here in the video with this theory
[link] but, what that video is saying that Star and Marco are going to be together and there is some hints like their dimensional scissors. The scissors in the show represent Star and Marco. Like in the episode "Running with Scissors" when older Marco is holding his scissors Star tries to convince him to come back and when it shows his scissors they were closed and then he opens them. They both reflect Star and Marco being apart and coming back together again. Sorry that this was long, I have been thinking about this since Monday. ^^
