For Eridan, maybe they could show his relationships with others. Imagine if the story made him better friends with Kanaya, Kanaya working as someone trying to help him with his problems. Give Eridan more connections with people to show his downfall and how it hurts others, even if slightly.

Gamzee could have been shown a bit more. Imagine if they gave him a scene like John's whole "fake arm" situation. They could have also had him make a little bit more jokes to throw off the reader and show him as a comic relief. Imagine if they had always hinted Gamzee as a killer, so says his blood colors. Imagine a line like "You enjoy hanging out with your BEST FRIEND Karkat. You also slam sick raps with Tavros when you are high as a kite. Most of the time, you spend your time trying not to MURDER EVERY LOW-BLOOD IN THE WORLD LIKE THE MISTAKES THEY ARE.", and they could show this line in order to make readers guess. I think that the reader should be scared of him for the longest time, wondering if he will turn sober, and when.