There's just a lot you can do with side characters like Gideon, Robbie, Wendy and McGucket. Also, I would like to see Ford being a badass and saving the rest of the Pines as well as Gideon's reaction to him. We already got enough development for Dipper, Mabel, Stan, Ford, Soos and Pacifica. So I'd like to read the side characters development this time. Elaborating the characters I said first, in terms of development:
Robbie's questionable, Love God might be enough for his character.
Wendy could use a story centered around her, this is something the writers wanted too.
Gideon should have one story that involves Ford, the figure he revered throughout the series, plus knowing that he's a Pines would be something to see.
McGucket's reunion with his family and gradual recovery should make for a nice comic. Although the finale and Journal 3 might be enough really.
I'd like to include one more though, and that's a Mabel and Ford centric story.
So that's three characters that could use a story, Gideon with Ford, Wendy and Mabel with Ford. And after that could be a Stan and Ford arctic mini-series.