I like the idea here, but I feel like you could take this even further. Like, for example, in a lot of Greg Martin's art, Eggman is gritting his teeth furiously(kinda like in that last picture, actually). You could also remove his gloves, as a lot of Martin's work tended to not give him those. You could also exaggerate his brows. You're kinda already on the way there, but I feel like you could really accentuate that look to make this much more impressive.