The ending made sense... but it left me a little underwhelmed. After a bit of thought, I have decided that this could be seen as a more of an open ended conclusion, only resolving the Aku story line. The story can go where you want it to go from here. If you wanted the melancholy ending, leaving it where it is makes sense. I prefer a happy ending, so there are a few ways the story could go to be more fulfilling. Just wanted to throw these options out there for catharsis and letting go of this great series.
Jack didn't get his happily ever after and has lapsed back into his old wandering ways. I prefer to think of the lady bug as a way to show Jack that Ashi protected the nature that she loved in the end, but also as a reminder of the hope that Ashi revitalized within him.
Here are a few ways that the story could conclude.
Jack now has 50 years worth of knowledge about mystical places and artifacts on earth that have existed even before his time. Jack may begin a new quest to find a way to wish Ashi back (Dragon Balls anyone? lol). After all, she may not physically exist, but Jack's memories and experiences in the future were real, otherwise he would not have returned. This is the simplest option.
With his vow fulfilled and Ashi gone, Jack has no reason to stay in the past. He might abdicate the throne leaving his sword behind and go on the same quest, only in this scenario to wish to return the future that he knew and loved cleansed of Aku with everyone he knew restored, including Ashi. He would understand that they would have no knowledge of him, so he would begin a new journey meeting everyone once more with new origins and new stories. This one would be a little more complicated necessitating a season 6 and culminating with Jack having to find his sword (hidden away for centuries) and defeating a new (though less threatening villain). This would endear him to his friends once more and allow him and Ashi to return to his home to live out the rest of his days in peace.
Option C (my favorite)
Jack goes on an epic quest in the past to bring Ashi back. Along his journey he meets many new and interesting characters. Among them, a masked woman trained to be a ninja that was sent to break the emperor's bloodline. Of course Jack defeats her and earns her respect. Understanding that her failure means death and she has nothing more to live for, she follows Jack on his quest to find his way back to Ashi, eventually growing to care for him. Overcoming a final obstacle, Jack and the ninja finally find what they are looking for and Jack asks for Ashi to be restored. Jack is informed that the future that he knew no longer exists, therefore she cannot be returned. Stricken, Jack collapses to his knees. Attempting to comfort him, the ninja removes her mask only to reveal Ashi's face. Jack doesn't understand how this is possible. The person protecting or overseeing the wish granting object states that while her future self may not exist, her soul would have been reincarnated from previous lifetimes. This Ashi would be one of those previous lifetimes. Jack would make the only reasonable wish, to share his memories of the future with her. Thus, Ashi would be restored and they would return to the palace to finish the wedding that was previously thwarted by the space time continuum.
This one even included a reference to their conversation involving becoming memories. I like it! One hour special anyone?