Well, if they would make a new series, it should not be about a dark avatar. There can only be one at the time, and the idea is very lame. And about the first Avatar, that doesn't seem good to me. If they do there will be no mysteries left in the series. I just think IF they are going to make a new series, it should just be about the gAang. There are plenty of things that can happen.
Looking for Zuko's mom.
Looking for Airbenders. (but if he gets children with Katara in the future, can they also become airbenders? if so, they don't need to look I guess.)
Azula escapes, and is busy terrorising people or chasing the gAang.
There are probably some Fire Nation soldiers who are still loyal to Ozai, and try to free him, or they join Azula if she escapes. Or they are just being rebels who are attacking the good soldiers or villages.
More about the OWL, and Irohs trip to the spirit world.
This could make a new series of 20 episodes I think. And maybe also a new villain.