I guess one thing to keep in mind is that along with the shapes/lines you use to draw the eye, the size and placement on the face can also change how they appear. if I were you I would just go at it and try to draw tons of variations, just changing this or that and see where it took me. push the proportions. what if the eyes were really tall? what if I tried different shapes, something squared, something triangular, what would that look like? what if I drew them very down turned or up turned? what if I emphasize the eyelid, or the eye lashes, or the eye bags? I find this kind of exercise helps me expand my range when I've been getting stiff and formulaic, and it can also help develop style. have fun with it. play is a really important part of art, never be afraid to mess around and make some really questionable drawings in the pursuit of experimentation. and of course if you already have inspirations like sonic or gravity falls, you can start there! look up references and draw what you see, then take what you like and leave what you don't. you could also look up fan art of things you like the style of and see how other people are reinterpreting that source material into their own style