I believe I did mention how Dialga and Palkia's fighting made sense due to how they represent time and space and how something like that wouldn't make sense for Ho-Oh and Lugia.
I don't think replacing their franchise's mascot is that easy. Even though I feel like the writers have been giving Piplup way too much screen time in DP, it couldn't replace Pikachu as the mascot for the whole franchise. Pikachu is one of, if not the, most well known Pokemon out there. As Davdix mentioned, it has been a part of every Super Smash Brothers game and it is catchable in every game, even though it's only native to Kanto. That and the fact that Pikachu, like Ash, has been in every single episode of the anime series, causes me to believe that they will never replace Pikachu.
I don't think that it's as easy as you lay it out to be. If it was, then I think that the writers could have done it by this point. Besides that, I think that this other Pikachu would just turn out to be the same as Ash's Pikachu, just like this replacement trainer would turn out to be like Ash since they would be doing the same exact thing with the gym battles and trying to get into the league.
That would be a new twist on the series and it would have Ash as the main character still. However, I don't really think that it's too likely to happen, but it is a nice idea.
I believe that the series has always been a thirty minute commercial for the games. Even back in season one, it's main purpose was to promote the games and I believe that it is still the series main purpose.
While I do understand where you're coming from, along with others here who have mentioned their dislike for how long Pokemon has gone on for, there is just one thing I believe people have overlooked. Everyone who is on this forum has to be at least thirteen or fourteen to sign up. Pokemon is aimed at children not much older than twelve. In other worRAB, we are far outside of their target audience and as long as the kiRAB enjoy the series, they will continue to handle it the way that they have been for years since that has proven effective and successful for them.
Well, considering how long Ash has been the main character, he is fairly well known aspect of the Pokemon franchise, similar to Pikachu. I might be one of the few who isn't afraid to admit it, but I don't mind how long Ash's journey is and how long it has been. I love Pokemon and I have enjoyed watching the series for over ten years. I don't see that changing anytime soon.
I honestly wouldn't be surprised if Ash's journey is still continuing in 2020. I actually think that would be kind of cool. I think that it's at least a possibility. I don't think anyone who watched back in 1999 believed even for a moment that Pokemon could last over ten years with 600+ episodes under its belt. I certainly didn't.
I understand what you're saying about the new cast, but it comes down to what the writers want to do. In all honesty, I don't really see them giving up Ash and Pikachu at all. They could have done so a couple of times, like before Johto started or even when Ash defeated the Kanto Battle Frontier, but they didn't. Considering he has been the main character for so long, I see that more as a sign of how he's here to stay, rather than a sign of how he could leave.
I feel the same way about Ho-Oh. It has an interesting storyline in the games and all, but I didn't think that it was a huge part in the anime series. I thought that they just used it more as a plot device to show how special Ash is, even though the last time Ash saw Ho-Oh, May was the one to see it first and I never understood why it showed itself to a large group of people either.
Anyway, as for the actual topic of this thread, it could be that Ho-Oh and Lugia may not the main attraction of this movie. I just came to this idea after reading one post in another forum. I completely forgot the main rule in most of the Pokemon Japanese trailers: they are usually, if not always, misleading.
I vaguely recall seeing Mewtwo in the trailer for the second movie, even though it wasn't there. I remeraber how for the Deoxys movie, it landed in a desert area instead of at the North Pole. For the Lucario movie, Lucario was shown to be trapped in a large crystal, instead of the staff that it is in during the early part of the movie. Finally, for the Giratina movie, there was a part that showed Regiggias' arm coming up from the ground and it barely had any role in the movie.
So, it is possible that Ho-Oh and Lugia could play a part in the movie, but it could also be that they are there to promote Heart Gold and Soul Silver, which come out in two months in Japan, another aspect I forgot to consider, and other more. I also still think that they'll showcase some kind of fifth generation Pokemon in the movie.