Honestly ... handling the end of JarCo ending poorly and either make Jackie look like the bad guy, or make it far too convenient for StarCo.
ATM ... if you look at him as an individual character Marco is NOT worthy of Star and he's certainly NOT ready to be together with her. Season 2 was so devoted to Star's growth and development that bluntly ... Marco's been left in the dust. He feels more like a side-kick than he ever has in the past. He needs a chance to catch up and Season 3 AND a well done, slightly bitter, JarCo breakup (where Jackie breaks up with him for herself and calls him on HIS failings) can not only salvage Jackie's character and promise better things for her in the long run, but also serve as a catalyst of growth for Marco (and even potentially help Star grow a bit more too, if handled REALLY well).
It may slow down cannon StarCo for a while yet ... but that relationship will be far more rewarding when it finally does happen, because while Star has certainly proven she has earned StarCo ... Marco has not yet. :D