Ok, this is a rather large thought. I will try to make it as coherant as possible. Warning: SPOILERS for any who have not read the series all the way through.
So, I really liked Star of Deltora. The characters are great, and I like the new power trio (though the originals will always have a special place in my heart. I love how it ties the world together, and further, I think it is really cool that this world is getting a new addition to it, so long after the initial release of the original series.
Also, it's really nice to have some more details added to clarify things from the original series. Like how Thaegan differs from Fellan-blooded sorcerers, because she probably follows in the same tradition as Tier's unnamed Mom, given that she is the only one to give birth to a spellwarped child aside from Thaegan that we know of. And it clarified why everyone seemed to know about her blood weakness when she very obviously hadn't died yet. She had a predecessor. And it potentially explains where Malverlain got all that "Dark Lore" from (Bar-Enoch?).
But, as has been pointed out by another user, while the series ending was very good as a story, it was not very good as, well, an ending. I personally thought there were too many loose ends hanging around. For example, all the mentally diturbed people in Del Palace, who have apparently been that way since the end of deltora quest series two. Which is really depressing. The one thing that could have helped them, the Staff of Tier, turned out to also been rather homicidal in and of itself, making it difficult if not impossible to use for good, and thus leading to the necessity of it's destruction.
And then there's the newly introduced mysteries. What is the isle of dreams? what is the culture of the Isle of Jade like, and do they have magic? And what is the Crown of Serpents/can it be used to help undisturb people?
Also, we still don't know where Polypans come from, and for that matter why there was a steam boat in the river broad when the average tech level of the Ocean of the South is... not steam-powered. Is there some sort of random steampunk island?
Anyways, for these reasons and more, I really think the Star of Deltora not only deserves a sequel, but needs one. Any thoughts?