The Pokémon Company have released a bizarre video today that shows some of the most peculiar scenes from the series, with a promise to reveal something more on the 19th of July.
This latest trailer is one of the most unusual things ever connected to the Pokémon franchise. It shows a montage of several weird moments from the series, including the event where Celebi takes you back in time in Pokémon HeartGold & SoulSilver, flying through a rift in Pokémon Omega Ruby & Alpha Sapphire, Dialga using its Roar of Time attack, the dancing Clefairy sequence from Pokémon HeartGold & SoulSilver, Deoxys in space from the Delta Episode of Pokémon Omega Ruby & Alpha Sapphire, the mysterious old man from space in Pokémon Sun & Moon, the Distortion World from Pokémon Platinum, the Arceus Event in Pokémon Black 2 & White 2, and the arrival of the Ultra Beasts in Pokémon Sun & Moon. The trailer ends with the Unown spelling out the phrase "Nani ka ga okoru?", which loosely translates to "something will happen?"
So what does all of this mean? The running themes of the trailer appear to be time travel and outer space, which includes the presence of extraterrestrials, like Deoxys and the Ultra Beasts. This might possibly be the announcement trailer for updated remakes of Pokémon Diamond & Pearl, as those games have a time & space theme. Palkia and Dialga also has power over time and space. The main reason why it won't be a Sinnoh remake announcement is that we would have likely heard about it during E3, rather than in a random trailer a few weeks later.
The most likely answer is that we will be finding out more information about Pokémon Ultra Sun & Ultra Moon. We likely won't be hearing about a new Pokémon game, considering that the Pokémon-themed Nintendo Direct and E3 only happened a month ago and any new game reveals would probably have happened there.
It is possible that we will be finding out more about the Ultra Beasts and their role in Ultra Sun & Ultra Moon. The mascots of Ultra Sun & Ultra Moon are same as from Sun & Moon, except that they have been corrupted or possessed by Necrozma. There is speculation that Necrozma is connected to the Ultra Beasts, which would tie into their increased presence in Ultra Sun & Ultra Moon.
We will hopefully find out what this reveal means on the 19th of July.