Mistborn can't be over--the very ending is a beginning for a continuation. Think about it: a new world, a new god, no rulers, no laws.
You see, i think that Preservation and Ruin were merely lesser deities. How can we know this? well, we still don't know where the prophecies originated from. It couldn't have been from Ruin--because he wouldn't alter his own writing. Nor Preservation, because he would have the sense to write it in metal, or atleast know that writing the prophecies was pointless. Another thing, Ruin would have tried to kill or control Sazed, and Preservation would have confronted him in the shape of the mist spirit...they did not. I wonder if they even had a clue.
Perhaps they were the SonGods of a Father Deity. Perhaps just Gods of the world amongst other gods of other worlds. Or universes. I just don't believe they were the top most gods.
They might not have even been gods. Sazed said it himself. They were the two aspects that a god would have. So if they weren't god, and just powerful beings...who is God?
No, this is definitely not over.
Oops! almost forgot. Since Dear Vin did infact merge with the mist, i will be expecting my dubbing soon.