Personally, I really liked the ending. I like how it broke the traditional mold in terms of
SPOILER: Highlight text to read: the main character not marrying the princess, which is what I expected. The ending also really flushed out Slaine Troyard's character, which I really appreciated. I was having a real hard time understanding his actions until then.
I don't see too much in terms of a sequel, but a prequel (following Marito's story, perhaps) would be really interesting. We could take a look at Heaven's Fall. Although, considering the history, that make for a rather dark anime. I bet that they could pull it off well though. What do you guys think?