>just changing animation quality, physics, attack methods, and level design

So...basically everything about the game but the genre?

I mean, isn't that the exercise? I love MTJ's Bubble Bobble, but it is a simple game. If a "Rip O Matic" took Bubble Bobble and replaced the bubble blowing dinosaur with a sword-wielding samurai (but kept the levels, speed, etc), it would be a different experience. If a Rip O Matic created 500 new levels, or cloned the game and replaced the dinosaur with a bubble-shooting Astro Boy, it would be undeniably derivative.

But there is some boundary line on the level of effort where a game becomes its own thing. Fortunately for the industry, the number of parameters that can be "tweaked" grew exponentially with each console generation.