I feel like they could of resolved some arcs better
Terezi and Vriska should of been on a more equal footing instead of having Terezi feeling overdependant on her.
Karkat should of done something more important in the finale instead of just become a joke, he doesn't have to be leader, but reducing him to a gag feels cruel considering his self-loathsome nature.
I feel like we should relfect more on the fact that everyone from John and Roxy's og timeline is dead I think we brush over that too much.
And the fact Jade was alone for years.
Oh and more trolls should of fought HIC, especially Karkat.