I feel like Dialga and Palkia won't be in this set. They would probably opt for Xerneas and another Gen 6 mon to fit the theme (and allow for a Blend FYD)
We don't know that Palkia and Dialga are in the set yet, if my memory serves me.
While it's way too early to say for certain, I feel very confident that Palkia and Dialga will be in the next set. They were printed in SM5+ Ultra Force, and, as is the usual with Japan's mini-sets, the unique cards from those mini-sets are funneled into our next English set. The alternate type prints of Palkia and Dialga have unique FA and RR designs, which essentially means they can't be released in some kind of collection box.
The reprint of Silvally-GX in Ultra Prism indicates to me that TCPi has no qualms about releasing exact/functional reprints of cards between two subsequent sets.
Finally, adding Palkia and Dialga to the Kalos-inspired set would help drive the narrative that TCPi seems to be building with this block - that is, the "
Ultra Wormholes creating connections between various times and spaces." Essentially, each set in this block will represent a mix-and-match of various generations, meaning it wouldn't be out of the question to see Gen IV Pokemon being put in a Gen VI set.
Of course, I could be wrong in all of this. If anyone has any theories on how TCPi is going to release all the alternate type prints from SM5+, as well as the RA, FA and RR for Palkia and Dialga, that doesn't involve putting them in Forbidden Light, I would genuinely like to hear it.