With how long a wait Avatar 2 is going to be, I'd suspect they will have struck the iron way to late. If it keeps on getting delayed like this, chances are no one will care. It certainly hasn't left a cultural mark as sizable as other phenomenons such as Jurassic Park, Star Wars, Titanic, etc.
I think making a sequel to it could be about as successful as a sequel to ET. If you remember, Jaws 2 did under half of what Jaws did as did Jurassic Park 2.
Sequels to phenomenons do not equal another phenomenon.