There's so many unlikely sequels that came to mind when I saw the thread title, but Okage's the one that popped up first. It was my first JRPG that wasn't Pokemon, so the gameplay felt fresh to me. In retrospect, it was probably tedious, but I loved it as a kid. The art style's what drew me in; it was unlike anything I, or anyone else that had never seen literally a minute of any Tim Burton film, had ever seen before. However, I liked the setting, I liked the characters, and even now that I know that the game probably wasn't all that special, I still want to see it return.
Mischief Makers is a close runner-up, though, and unlike Okage, I can still vouch for its greatness.