We have Shadow versions of Mewtwo, Lugia, and Dialga*, so Gen 1, 2, and 4 are covered. Let's say one Legendary per generation. And bonus if you don't pick Giratina or Necrozma. Oh and double bonus if you have a lore concept for how they went dark: corrupted by an org, absorbed dark energy, lost control of its powers.
*I know that Dialga is Primal Dialga not Shadow Dialga, but that's just in the localization. In the Japanese version Dialga is called Dark Dialga, which is the same term used for Shadow Mewtwo and Shadow Lugia.
by heart-station