Here's a theory that I've been having lately, and I know it's been covered in the past, but since I do not follow many PokeTubers, so I do not know how much detail exactly this theory has gone through with other Tubers. Beware.
I know the "Ash is in a Dream" thing is really popular in the world of Pokemon we all know and love. I haven't seen too much detail go through for this, so here goes some more possible details that this could very well be a dream.
It all starts in the original Red/Blue/Green games. Before this time, there were no Pokemon at all. Well, in our world. In the official anime and games, it specifically states and stated by Oak that there may very well be more than 149 Pokemon. They do not know everything about Pokemon, even though they have been around as long as the human has. Back then, 149 Pokemon was more than enough, before the Pokemon Mew and Mewtwo were discovered. Now we advance the American timeline up some years to the release of Gold and Silver, released in the year 2000, the turn of the new century. Here is where the theory explodes. With the release of the new games, there were another 100 species discovered, along with another type: the Dark type. Birch, professor of Johto is obviously older than a teenager, at least 20 years old when the games and the anime take place. Let's say Birch is 20. This two decade old professor has been in Johto for 20 years, and yet, all these new Pokemon are new to him (does the last sentence sound suspicious?), yet there are new species RIGHT OUTSIDE THE TOWN. The new Dark type was something no one knew existed before the games of Gold and Silver, and if you think about it, the move "Bite" was not super effective against Psychic types in RBGY, but is in the new GSC. For future reference, allow me to refer the new Pokemon as the "Pokemon Effect," so I do not have to add any unnecessary info. In all games RBGY and newer, the Pokemon Effect has taken place in every generation, that is until BW2. In BW2, the Pokemon Effect is still active, and is even stronger now than was 12 years ago in 2000. In BW2, there is a thing called "DNA Splicing." which if you live under a rock, you know how the DNA Splicers work, fusing Kyurem with either Zekrom or Reshiram.Now we start so see even more details in the Pokemon Effect: Fusion. Now we advance to the release of X and Y, in 2013. The Pokemon Effect is not as strong in these games as in previous games, but in return, there's yet another addition to the Pokemon Effect: Mega Evolution. According to games, there has been signs of it, but never seen before now, along with the new fairy type. With the release of ORAS, there are even more Mega Evolutions to the Pokemon Effect, even though it took place before XY.
If you ask me, the Pokemon Effect and the new types show something, but what is it? Could it show that this is all indeed a dream?