A Sequel Isn't Necessary
Though hinted to, the concept of the movie was that Happily Ever After wasn't as Ever After as first believed. Following that line of reason, the end tells us that nothing changes simply because they were victorious this time, doesn't mean that now everything is happily ever after again, and sooner or later lives would be in jeopardy, the nine kingdoms would have more problems, just like any society, just like any life. Problems happen. It's just what they do.
Besides it's rare to get a sequel to a miniseries, and even rarer that is be better than the original.
I always took the end as a continuation of the, "Happily Ever After wasn't so Ever After As we Had Hoped."