What if Pokemon had zodiac signs?
And you could put different ones on your pokemon and they'd react differently with the opponent's, and if it was a strong reaction it would notify you and you'd do less damage or more damage or something
Or each one of them could be associated to a type (Aquarius water, Libra fire, stuff like that) and they could do more damage

'Course for legendaries it would already be set, because they were born at a set time via lore etc