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Alright, I've done all I can. Mega evolution isn't implemented in the game (for the player) yet, neither is Dragalisk as a playable mon, but all the other unobtainable mons have dex entries and are battle ready.
I looked around in the scripts as well for the postgame. The script on Mt. Zodiac with the nurse blocking the entrance saying the cardinals are in a meeting doesn't have a variable, which means there's no way to change it. The lady blocking the cardinal's home in the ice village also has no variable, however, the actual temple leads to an entirely different map, which is at the moment.... blank. Same thing with the cardinal temple in the fire mountain village. HOWEVER, in the primal town, the cardinal temple interior is already mapped out (because there was an event inside earlier in the game). There is an event for this Raikou wall, which when triggered seems to open.
So presumably the next update will involve the ending of the cardinal meeting on Mt. Zodiac and the return of the cardinals to their respective temples, whereupon you will be able to find and capture the legendary dogs perhaps. There's a remix of the legendary dogs theme in the files, they have pokedex entries and they have mega stones in the game.
Well I'm done searching around, can't seem to find much else.