I'm actually upset about this now. I wanted to see how Zuko would turn the Fire Nation and restore it's reputation, how the Water Tribes would rebuild, what happened with the Dai Li and government in Ba Sing Se, what will happen to the Air Temples, what will become of the Avatar Cycle, and most importantly, what happened with Zuko's mom! I think it is dumb to have a series 100 years later.
I'm sure they writers are more than capable of explaining these little bits of information. I mean, chances are, the new character will probably visit each of the lands. Therefore, I'm sure they'll touch on some of the major events that took place there.
Also, I'm pretty sure most of the younger characters will still be around in some shape or form. I mean, if Aang was "trapped" for 100 years in ice and his friend Bumi was still around even after all that time, I don't see why they couldn't have a select few other characters live for that long.
Also, I'm pretty damn sure they're would be enough references/cameos of some sort thrown around.
Overall, I'm curious to see what they'll do.