Give your characters more contexf. An example: if a character was in a mission that required trekking through the jungle, what would they wear, what gear would they pack? Let their appearance tell a story. We're they sweating? Maybe they underwater. Maybe they just got done painting a house. Maybe they were in a bar a little too long and now have two black eyes and a broken hand. Be specific in the design. As the creator, you know everything about the character. As an audience, we know nothing. Educate us with the visuals and the design.
Here's a tip. The best character designs are the ones that reflect who characters are on the inside. Use the designs to communicate the personalities of the characters. Take Loki in the first Thor film. Loki in that film has a rather large inferioty complex and this is reflected in the design of his armor. His green cape, his armor and the long curved horns on his helmet make him look like a literal insect that his brother Thor can squash.
We're working in vastly different styles, so you'll have to decide whether or not my advice really applies to you. However, I think some of the design tips I picked up from various educators have universal applicability, and could help you take your designs up to the next level.
I would focus on making your silhouettes more recognizable. If you strip away all other features, what does the character's silhouette in pure black look like? This helps each character read at a glance. While color palette, details and other features are important, the first thing we tend to see in designs is the overall 'shape language' of the character. I think their powered up / spirit forms are fairly distinctive already, so there's not much to say on that front. But I feel like your human silhouettes could use extra emphasis to drive home their personalities and roles. Because you're using a very simplified style, silhouette becomes even more important because it's the most expressive tool you have available.
If you identity the silhouettes of these two characters:
You'll see that they share a lot of similarities:
To compare, I'll take an example from your favorites gallery:
Note the extreme variability and expressiveness between each character's silhouette. Even the two boys are instantly recognizable despite being the same basic size due to variations in the silhouettes of their clothing, the angularity of their hair (one with more stray strands than the other,) variations in the legs, etc. Notice how the artist uses folds and creases in clothing to exaggerate character qualities, or accessories to provide added points of interest. There's a varied sense of weight, expressiveness, sharpness, etc, all of which is conveyed by silhouette / black outlines alone. By comparison, your characters have the same legs, same knees, similar shoulder curves, etc. Additionally, most of the characters you've posted here are also wearing very similar clothing: basic t-shirts or jackets, and trousers that differ only in color and small details. I feel that the top half of your characters are more visually engaging than the lower halves especially as a result of this.
Some of this could just be the result of using very basic poses (a crime I'm guilty of, too) but I'd argue that characters should look distinct in *any* position. I.E, if you were to pose ten characters in t-poses, they should all read as distinct from each other in immediately obvious ways.
Consider this Bruce Timm piece:
Note that although the characters are all running and facing the same direction, none of them could ever be mistaken for another, even if all the surface details and features were removed. Their highly exaggerated silhouettes make them recognizable instantly. In particular, note how Robin's legs are a lot more rounded and softer in structure than Batman's, reinforcing our impression of him as a less 'angular' character. He's literally not as 'sharp' as Batman, so every element of his design reinforces this impression of him as a softer counterpart, right down to the calves. Also compare the characters' chins. Batman's is a huge square shape (implying reliability and groundedness, stability and stoicism,) while Robin's is rounded and pudgier, boyish, more open but less tough. Batgirl's is razor sharp, while Nightwing's is something of a mixture. The greatest similarities in silhouettes here are between Batgirl and Batman, but the masculine / feminine shape language helps to offset most of them, while her exposed hair and half sized cape signify her as a sidekick.
Even within this image, there are lots of sublties in the poses as well. Batman runs with a sense of balanced speed and precision, Robin has a sort of headlong skater's pose that's full of wild energy, Batgirl has a lighter step, and Nightwing is sturdy but with a stiffer stride. In other words, they are literally wearing their hearts on their sleeves: the internal and external worlds are conjoined in this type of stylized art. What is within is also without.
All that said, my suggestion is to read up on character design shape language theory (just plug 'character design shape language' into google and dive deep, taking note of how personality can be conveyed through the usage of shapes alone.) Identify your characters' personality traits and try to embody them visually across the entire design, remembering to exaggerate as needed. Refer back to shape language theory when designing each element of each character -- a jacket (in stylized art at least) is never just a jacket, a hair cut is never 'just' a hair cut, etc. There are numerous ways to make each element representative of the character's personality and a wide range of exaggeration / stylization techniques. You're already doing this instinctively to some extent, so I think it's just a matter of pushing a bit harder in an exaggerated and dynamic direction and really seizing on how to convey each character's heart as a visual impression.
And finally, study clothing. Immerse yourself in fashion, sketch out lots of different outfits, and try to vary getups between characters as much as possible within the confines of your setting. Draw baggy trousers, shorts, skirts, dresses, belts, accessories, etc. They can add a lot of flair to otherwise stiff designs and give them more identity.
But all that's just like, my opinion, man. Good luck and keep drawing!
When I make my own characters I like to consider how they would "function" and what features and details would make sense for what they are.
Take fx. your third character. The lower part of his legs and his feet are pretty big. They look heavy too. So he would most likely have some really muscular thighs in order to move "normally" and to carry the weight.
I can also see you're showing their personalities in their poses. One looks a bit shy and so on. I like that you're doing that but I think you could exaggerate a bit more. Make it more clear what kind of person they are.
And lastly I think it could be interesting to see some more dynamic poses. Maybe not their "human-self" but their hero form. Just like you're showing their personality you could make the heroes show off their powers a little bit.
I've been trying to do more relaxed and dynamic poses, but it's a bit hard when I try to find references of posed that look natural and the fact I'm still learning to draw poses; it's a bit of a struggle.