Obviously, the generation 8 games will come with a new set of box legendaries, which will stand at the center of the game's story. But what could these legendary creatures stand for? Similar to the weather trio, or the Pokémon of life and death, every legendary has something they are the master of. What's left for the new legends to master?
What about chaos and order? I'm not sure we've had that one before. Would be bonus if the flagship legends were not dragons for once, as much as I love the dragon type I feel like we have too many legendary dragons right now.
Knowledge and ignorance? That seems like a possible pair. I feel like they would both be psychic types.
Pestilence, war, famine just to complete the four horsemen of the apocalypse (We have death already.)
What about pokémon based off of the seven deadly sins or seven heavenly virtues? I mean, I know that seven new legendary Pokémon is kind of a lot, but it could be cool.
They could copy Huntik and go Legendary Pokémon of mind, body, and spirit. Or maybe positive and negative energy since one of the main rumors going around right now is that the next games will be called Pokémon Plus and Minus
I have no idea what kind of themes they're going to want to tackle with Gen 8, but I would love to have the legendaries be based off of mythological creatures from wherever the next region is going to be based off of.