Back in the late 60's when I first started to ride, the 250cc bike was the "cat's meow." . . . a very proper bike on which to learn. Why do you seem to hate your Ninja 250 so much? Depending on your answer, perhaps you are ready to graduate to a more powerful/responsive/less forgiving bike. As far as "looks," yes I prefer the Ninja 500 to the 250. But I aslo prefer the 636 to the 500. But first things first (for safety's sake) . . .
Back in the day (60's-arly 70's) the choices ranged (in cc) from 70 to 90, to 125, to 150, to 250, to 350, to 550, 750, 1000 etc.