Seems like the title of Movie 9 has been released. According to
PokeAni, it is titled "Pokemon Ranger & the Prince of the Sea" (pokemon renjyaa to umi no ouji), featuring Kyogre as (one of) the legendary pokemon.
A 30-sec TV trailer has been released, which started out in some sort of research center, possibly inside a submarine. The narration reads 'Far away, somewhere in the sea, a legendary pokemon that no one has seen sleeps'. We then see a school of Remoraids and Luvdiscs swimming by, followed by Kyogre's appearance.
The scene next shifts to a coastline where a golden-haired boy is seen using the Capture Styler (the Pokemon Ranger device) on a wild Mantine. The boy has uttered the line 'Capture, on!'.
Ash and Pikachu then appears, with Ash asking 'Is that a Pokemon Ranger?', and Pikachu answering 'Pikachu!'.
The Pokemon Ranger next clings onto Mantine as if it's a glider, and glides across the sky before diving into the sea, during which he yells 'I'm counting on you, Mantine!'.
The trailer ends with some mysterious orb falling to the sea bed, bringing up the title 'Pocket Monster Advance Generation Movie, Pokemon Ranger and the Prince of the Sea'.
What we know now includes:
1) Ash is still the lead protagonist
2) The movie takes place at the sea
3) Kyogre is possibly the main legendary (though like Mew, there could be another Lucario type of hidden legendary to be revealed later)
4) The golden-haired Pokemon Ranger possibly owns a Wailmer (two posters have been released, one of which depicts Pikachu on a Wailmer, and the other the Pokemon Ranger next to a giant Kyogre)
Coronis~