This is pretty interesting, but unfortunately I am not artistic so I can't show my suggestions.
For simple designs, I think having a logo of their starter monster replace the element logo is a good alternative. For the headwear, it can be similar to Ash Ketchum's hat, but replaced with the element logo, or a mini starter monster image.
For eyewear, the simple design can be the geiger counter in the example, but instead of modern looking, it will have some elemental highlights like a wave for Water, flames for Fire, four pointed star for Light, and four leaf clover for Earth. The earrings can be something similar.
For the more elaborate ones, I think there are a lot of possibilities. For Light, they can have a halo, a spotlight shining down on them, white feather/angelic wings, angelic attire. For Fire, they can have tongues of fire on their head, pieces of fire/magma/rocks circling their feet, flame wings, leather attire. For Water, they can have an ice crown, standing on top of dolphins/whale/wave/ice shards circling their feet, ice wings, wearing stylish wetsuit/ice tuxedo/gown depending on what they are standing on. For Earth, they can have birds circling their head, squirrels/rabbits/animals circling their feet, feather wings, wearing sundress/shirt and shorts.
Honestly there are a lot of possible things, and we can even look to other games for inspiration.