That would be pretty cool. As long as it's better than their last battle, I would like to see May battle against Dawn.
I don't really consider Ho-oh a plot. It's more like a plot device to me. They mention it every once or twice a saga to show how Ash is a special trainer. The only time one could argue that it wasn't would be when it appeared out of nowhere in front of everyone, even Scott, back in the Battle Frontier. Although, I consider that more as a weird choice by the writers than anything else.
I doubt that the writers want to drop Ash or Team Rocket at this point. They could, but after being in over 600+ episodes, it seems to me that they aren't interested in doing so. Besides that, I figured that they wanted to keep Team Rocket around, not only for Jessie, James and Meowth, but also to have the always evil/present team in the series.
As for me, the trailer did look pretty cool. I don't really see why Lugia and Ho-Oh would fight with each other though. I could understand why Dialga and Palika would fight due to the whole space and time colliding theme, but Lugia and Ho-Oh don't represent drastic opposite forces like that. If the games are any indication, they seemed to fly around together before their tower was burned down.
Besides that, scenes from these early trailers are never used in the final movie. Even with the Giratina movie, the first few trailers were pretty different, such as a giant tornado carrying Piplup away and Regigigas having a more active/important role the in the movie than it actually did.