The first thing Terezi did in the comic was kill John. She murdered a child for no reason other than her own amusement. She was clearly supposed to be the "antagonist" troll and all of Vriska's story beats would have easily fit under Terezi's umbrella.
Similarly, Sollux was originally set up as the master of timelines and precognition. Remember his earliest conversations with Kanaya (which, btw, were fucking gold). He was the one who could hear the voices of the imminently deceased and knew the future. This role was spun out into Aradia, who aside from some pretty cool visual design and a couple interesting set-pieces was barely a character at all.
Then you have trolls like Equius, Tavros, Nepeta and Feferi. There's no need to extrapolate who would have originally performed their functions because they never did much of anything relevant to the story - besides being obnoxious meme characters. Gamzee was retroactively written into the story as having a major influence on the plot, but his role could have been farmed out to any of the constellation of characters (LE, Caliborn, etc) he became intertwined with, without losing anything of value.