Honestly I see what your concerned about. What about characters like Pokemon who are supposed to have fur but up untill now they have just used a plasticky texture. How will Pikachu look like? will he have fur like he's supposed too? Or will Nintendo continue to use that plasticky texture.
As for Mario I can see alot of room for visual improvement. His pants his shirt his cap his hair it can all be improved. No you don't need to change his cartoony appearance to improve him to better suit HD.
Kirby, true if they try to make him realistic adding fur and other stuff they could ruin Kirby. But much Like Pokemon will Nintendo actually seek improving the characters visual appeal or will they stick to their plasticy textures.
As for third parties, being forced to support HD will bring up dev costs considerably. Nintendo however will likely keep the next consoles dev costs down only slightly improving on PS3's visuals. I bet you Nintendo will still be the cheaper console to support as well as sport truly next gen graphics!
-JC7
"In God We Trust - In Games We Play " - Joel Reimer
