a persons ability to bend goes, thats something that ONLY the avatar can do and ONLY if s/he is pure of heart. as far as chi blocking goes, its something that is temporary unless they figured out a way to make it permanent.
i dont remember where i read it but i did hear that katara will die early in the series so anybody from the original series wont be alive in the later episode. the most well get most likely is aang in his spirit form and flashbacks of things that happened off screen and/or on screen