I'm always a fan of outdoor scenes. They're pretty challenging though.
Some ideas on making the dungeons more interesting:
In the dungeons, try to break up the background bricks. Have some windows, more lanterns/torches, broken bricks, embedded statues and other decorations, irregularities in the wall (e.g. columns that stick out, but not enough to get in the player's way), plant life taking over, scatches on the walls. If you want to go bigger, consider larger details to give the dungeons stories, like entire collapsed sections, tapestries and paintings, some hints of the dungeon boss, etc.
Try to relate the background with the foreground. Have the walls icy near the ice(?) spikes, have background furniture on horizontal surfaces, have the hanging platforms either suspended from the ceiling or show their supports, have columns supporting the ceiling next to gaps... I think your one floating platform already has supports, but they feel disconnected from the wall because there are no shadows.
Dungeon ideas:
Gross wet catacombs, full of moss and slime and who knows what else.
Obligatory lava level. Maybe the lava is being used as a source of heat for a metalworks? So, a combination of lava hazards and industrial-looking platforms.
Enclosed aqueduct system, perhaps with puzzles relating to floating platforms and water levels. Rather than the usual cramped dark sewers, this can be something airy, with large windows to the outside and relatively light colours and clean water. The water doesn't even have to be a hazard in itself.
Castle prison? A literal dungeon. Perhaps you have to navigate on top of cages and decorations, and below are enemies (guards?) that will easily overpower you (perhaps rather than killing you, they just capture you and take you back to your cell at the beginning of the level).
Alchemy/wizardry lab. Bookcases and glassware.
Some critiques for the existing art:
There's hardly any hue-shifting in your colours. Making the lights and shadows be the same hue as the main colour makes everything look a bit dull. Changing the hue is also a good way to add more contrast without having to go crazy with the values (darkness/lightness).
The coins aren't circles, they're two pixels wider than they are tall. If a more oval shape is intentional, then I think it would be better to emphasise it and make them very obviously oval, so that it looks intentional.
The bricks fade off to "black" too suddenly, I think. It doesn't look like stylistic fading off to me, but rather like a literal gap. Try making the inner bricks filled with the shadow colours, so it's more of a gradient from outside to inside. You may also want to make the shadow colour a little darker in the blue one.
The large red foreground bricks feel rather formless, I think making the shadow shape an inverse of the highlight shape would help. Or, maybe shrinking both by a pixel would work better, so that the bricks have a definite face instead of looking like a gradient.
The same large bricks also look a bit off because the platform corners are rounded, but inside the platform the bricks have perfectly sharp corners. I think having slightly rounded corners there would be better
The large background bricks feel essentially flat and two-colour because the highlights and shadows are so faint. I think you may as well reduce the backgrounds to two colours and show the bricks' rounded forms with the outlines rather than light/shadow.
I made this edit to address the brick shapes and brick fading, and then got carried away playing with colours :'D
I felt your main character was a bit hard to read, so I tried to simplify some of the details, highlight the important stuff, and give them a little bit more form. Not sure how successful I was, but hopefully it'll give you some ideas.