I think the movie will go something like this:
Ash and co. go to some peaceful town. The people then tell them a legend somewhere along the lines of many invading armies attacking the town. However, Victini used its Victory Star to win the battle by itself and keep the peace of the town (Think the fifth movie). Ash probably spots Victini at some point, who probably runs away (after having some misadventure with Ash and co's pokes.) Then, suddenly, a mysterious lightning strike destroys a building, and pokemon are mysteriously kidnapped. Ash meets some movie-exclusive person who is trying to find out what caused this using some super high-tech device.
Ash, hearing of the legend, tries to battle Victini, only to lose horribly. (Later, Victini probably appears again to play with all the Pokemon.)
Victini leads Ash, his friends, and the movie-only characters into some place that was massively wrecked by lightning, where they meet Zekrom, who tries to attack the group. Victini tries to fight, but one of the humans says it's not worth provoking Zekrom.
Eventually, Reshiram appears out of nowhere and attacks Zekrom. It was trying to stop Zekrom from destroying the town and kidnapping the Pokemon, but it goes too far and the two begin destroying stuff.
It eventually turns out that there is some gigantic tornado about to destroy the town, and Zekrom was trying to put the Pokemon in a safe place and warn the people about what was going to happen. Ash and friends understand, and feel bad that they misunderstood Zekrom.
The two dragons try to attack the tornado to no avail, with Ash and Victini riding on Zekrom. Ash and Victini jump into the middle of the vortex, as all of the main character's Pokemon help attack the tornado. It seems like all is lost, when a burst of energy comes from the tornado, destroying it. It is Victini's victory star, just like in the legend, and Victini and Ash have saved the day!
Ash and Victini fall from the tornado, seemingly to their deaths, when Zekrom comes in at the last minute, saving them. Victini and Ash thank Zekrom (Victini is probably telepathic), and the three touch the ground. Ash's friends + movie onlys are happy that everyone is safe. Resh and Zek fly in the sky together, and Reshiram and Zekrom fly away. Finally, Victini waves goodbye,and Ash and friends set off on their journey.
So, this is what probably will happen, or at least something along the lines of that.
Ash and co. go to some peaceful town. The people then tell them a legend somewhere along the lines of many invading armies attacking the town. However, Victini used its Victory Star to win the battle by itself and keep the peace of the town (Think the fifth movie). Ash probably spots Victini at some point, who probably runs away (after having some misadventure with Ash and co's pokes.) Then, suddenly, a mysterious lightning strike destroys a building, and pokemon are mysteriously kidnapped. Ash meets some movie-exclusive person who is trying to find out what caused this using some super high-tech device.
Ash, hearing of the legend, tries to battle Victini, only to lose horribly. (Later, Victini probably appears again to play with all the Pokemon.)
Victini leads Ash, his friends, and the movie-only characters into some place that was massively wrecked by lightning, where they meet Zekrom, who tries to attack the group. Victini tries to fight, but one of the humans says it's not worth provoking Zekrom.
Eventually, Reshiram appears out of nowhere and attacks Zekrom. It was trying to stop Zekrom from destroying the town and kidnapping the Pokemon, but it goes too far and the two begin destroying stuff.
It eventually turns out that there is some gigantic tornado about to destroy the town, and Zekrom was trying to put the Pokemon in a safe place and warn the people about what was going to happen. Ash and friends understand, and feel bad that they misunderstood Zekrom.
The two dragons try to attack the tornado to no avail, with Ash and Victini riding on Zekrom. Ash and Victini jump into the middle of the vortex, as all of the main character's Pokemon help attack the tornado. It seems like all is lost, when a burst of energy comes from the tornado, destroying it. It is Victini's victory star, just like in the legend, and Victini and Ash have saved the day!
Ash and Victini fall from the tornado, seemingly to their deaths, when Zekrom comes in at the last minute, saving them. Victini and Ash thank Zekrom (Victini is probably telepathic), and the three touch the ground. Ash's friends + movie onlys are happy that everyone is safe. Resh and Zek fly in the sky together, and Reshiram and Zekrom fly away. Finally, Victini waves goodbye,and Ash and friends set off on their journey.
So, this is what probably will happen, or at least something along the lines of that.