Honestly I'd like Jackie and Marco to form a strong relationship on screen, but then break up because Marco feels bad for Star and Jackie is hurt because Marco won't stop worrying about her.
Marco talks to Star and Star tells him how she really feels. Marco accepts her feelings and they kiss. Starco becomes canon.
Jackie is deeply saddened over losing Marco and she feels the pain that Star felt, and Star feels bad for her to some degree, but is happy with Marco. Marco still cares about Jackie deep down and tries to ignore it, he's happy with Star, but has moments where he misses Jackie. Star notices this at times and it bothers her. She eventually comes to understand that Marco still loves Jackie and she sees what's going on. She then dumps Marco who is shocked and doesn't understand. Star realizes that it's okay for them to remain the best of friends, if that's how he really feels, and knows that Marco and Jackie were made for each other, it's okay because that's a part of growing up. By the end of the show he and Jackie get back together, and Star goes home to take her place as queen of Mewni