You could use some more off guard expressions-Some features react quicker than others, the eyes are only partly open when surprised. If you find a place to do so, maybe double on some facial reactions while treating the parts of the face like they’re not related. You can do this by thinking of a plant, a general position and then have things within the plant change. It’s ridiculously counterintuitive, but maybe you could take a drawing you already made and make subtle modifications while keeping aspects of the original drawing. You could also do the same thing with animation itself and you could work with more than a single still drawing.

One general observation that you’ll find useful, a lot of animation tends to avoid small facial details. They’re good if you want something that changes how the viewer perceives a character’s expressions. It creates a sort of barrier between the viewer and the character and it’s something that constantly shifts the viewer’s judgement. If you give a character freckles or a scar right next to their mouth, no matter how they emote, the perception of it will be colored by the general look of their face. You have to be careful not to put too many flaws where you can’t forsee their impact.

And my last suggestion is to start breaking out of unnecessary symmetries in character design. It won’t necessarily be good for every character, but it’s a good thing to try and see where it works for you.
Here’s a number of ways I suggest doing it:
Slight differences in size (one hand slightly larger than the other)
Differentiated curves (one cheek bulges more towards the bottom part, the other the top part)
Subtle distinctions (One eye has a convex curve towards the nose, the other with a slightly concave imperfection on a similar curve)

Those are my thoughts. I hope they helped contribute.