I actually predict that Nefcy will deny starco from the fandom for at least the remainder of the season just to keep people on the edges of their seats. There will be some slow and steady starco developments, and Marco might slowly start to explore his feelings and slooowwwwllyy start to advance towards realizing that despite wanting things to stay the same, he might see Star as more than a friend - but it'll be countered by
a) reminiscing about the romance between Star and Tom, Tom growing as a character and truly leaving the "asshole demon ex bf who's manipulative and possibly abusive" role he played upon introduction behind him for good
b) Jackie, despite expectations, not getting dropped asap or by even mid-season, but instead staying on the show and as Marco's gf and their relationship progressing and blossoming and
c) Marco displaying clear discomfort at the thought of romantic advances from Star, and expressing that he wants things to stay as they were, and sees Star as a friend
Other predictions include Eclipsa growing into the next big bad, via appearing to be a fun mentor-like figure at first. Also more Mewni lore and more info about the other kingdoms.