Well one sequel series in Legend of Korra is worth noting. I found it to be really good but many argue its not as good as the original since it was rushed or had little downtime to flesh out the characters like ATLA did in Book 2 and especially Book 3. In that I'd argue they have a point but the quality is still up there.
Contrast that to the French Anime Code Lyoko and its reboot/sequel Code Lyoko Evolution. You know how ATLA fans hold M.Night Shyamalan in disdain over his live action movie adaptation? Well take that x10 with CL Evolution(won't say any more than that).Then there's Star Wars Franchise. Empire Strikes Back is widely regarded as the best film(still is to me) but its not just movies. Clone Wars Animated series is widely regarded as the best. Even more so than Rebels, although I thought they did a decent job. Was disappointed in the finale with regards to Thrawn(he deserved better) but otherwise it wasn't half bad.
Disney has several sequels slated to debut in the next 1-2 years: Wreck-it-Ralph 2, Frozen 2, Mowgli(Jungle Book 2), Maleficent 2(forget the title) and more. What should be talked about isn't sequels but prequels. Like wtf?