The last movie should be at the end of Sinnoh. I do not enjoy it as much when Ash nevers get far enough before they start a movie. But Arceus and three Pixies need to be in it. Heatran would also be in it as well. I already have some ideas. Arceus arises and comes down from the torn world. His presence awakes the three Pixies. Heatran is some what like Suicune and helps save the world from Arceus. Would Arceus be coming to claim the world? I'm not sure but it should be interesting!
Also, it's too bad that you can't go to the unknown cities in the DS game. The city in the 10th movie is awesome.