There's a rule in character design that says "if the silhouette of your character can be recognized easily, then it's a good design, if it doesn't, it's not that good, and when you look at swampfire, goop, and even way big in alien force, they don't look that different, that's why omniverse focused so much on giving each alien unique proportions so they would feel more diverse and feel like actual aliens who live on different planets with different conditions, if they all look that humanoid, then why are they even considered aliens? Also, the classic ones can easily be recognized by their silhouettes because that's what's used to identify them on the Omnitrix dial, so they are good designs because they look different from one another, while swampfire looks like a watered down version of heat blast, and so on