this has a bit of a Game&watch thought to it...were you aware of that at all? I say that because of the purely colored background which has a completly diferent sense of depth than the main characters.
maybe you could emphasize that too by making the chars 1 bit and animating them in a Game&Watch manner?
anyway, you got something going on with that background...honestly I think the sprites arent very special except because they contrast so much with the background, the contrast in the sprites themselves is poorly handled since the only truly contrasting color is the black, the rest of the shades look like they should be just tween shades not the keyshades like you are using them.
I think you need to study some more resources on miniaturizing features, for faces you should look for Megaman series games.....practically any of the spinoffs except for MEGAMAN ZERO is really good at that.
Most nes games are good at that too...you can check Shantae on GBC as well.
Probably GBA rpgs, except dont relay on the outfit details rather than the faces because they tend to use lots of templates.
for the general construction of the sprites you can look for Castlevania SOTN...there's something about the way those sprites are shaded that is very concise, frankly the GBA versions are a disappointingly poor translation of that, the new CV DS game has mostly poorly drawn main characters, but most of the enemies (since they come from SOTN) and some of the new summons are still good.
Strider2 for psx would be a great catch too, specially for the armored enemies in the medieval level. Heart of Darkness for PSX is great as well.