I like the idea of a movie based on the Ancient Culture that worshipped the Unown.
From the feeling I got while playing Gold/Silver/Crystal and FireRed, I came to the thought that they were quite advanced for their time. Much like the Egyptian, Mayan or Aztec cultures.
And this one doesn't need to have Ash & co as main characters. Introduce a movie-only character or characters and let it take place in that time. Perhaps flash forwards of scientists and archeologists trying to discover the "truth" behind recently discoverd ruins in the present time or something.
It could be partially based on the story of Atlantis. It could explain the reason why the Unown "fled" to a different dimention (not saying they did according to the "canon" information we currently have) and took half of the culture with them.
Yes, I quite like the idea. :'D
"We shall build a peaceful world... Just you and me..."