I think I'm gonna come up with some ideas for some additional old legendary encounters for the games that lack them.
Ultra Sun/Ultra Moon: Dangerous Ultra Beasts have the power to go into the main universe, why shouldn't the reverse be true?
Shortly after Rainbow Rocket, you hear reports from the Ultra Recon Squad that a strong Pokemon has appeared in Ultra Megalopolis. You go there to find it absolutely ruined, parts completely charred. This turns the city into a dungeon of sorts. This gives you the opportunity to catch wild Poipole (and maybe more?). At the base of the Tower you fought Ultra Necrozma in, you see Reshiram (Ultra Sun) or Zekrom (Ultra Moon), and then it fights you. Once defeated, the Ultra Recon squad says thanks and says it will rebuild in no time.
Once you return to the main world, Colress will be standing at the Altar and says "That Pokemon... I haven't seen that Pokemon in years. It must have came from an alternate dimension, as the one of this world is in the posession of the former champion of Unova. Wait... didn't the Ghetsis of Rainbow Rocket have a Zekrom(US)/Reshiram(UM)?" He does a quick check and confirms it as the one from the ruined timeline. "Facinating... but player, this means there is a rouge Kyurem on the loose. I cannot afford to let Ghetsis get his hands on it. Would you mind going to this alternate Unova and obtaining it?".
It's a yes/no prompt, if you say no you can go back to do whatever and Colress will stay there and wait. Once ready, you and Colress go through a wormhole and find yourself at the beginning of the Giant Chasm (the sudden teleportation causes a rockslide that destroys the way into the rest of Unova). It's much the same as it is in B/W 1, and you go and fight Kyurem. When you go back to Alola, the screen stays still for a second and Ghetsis appears. He says "so, they stole my Kyurem... well, two can play at that game, I've heard rumors of the power of the Tapus... what do you think, Colress?", which is foreshadowing for the postgame of the B/W remakes ten years down the line. You can freely return to the Giant Chasm, like with Ultra Megalopolis. After doing this, Megalopolis is rebuilt, and you can explore more of it.