As they are, there's nothing special about them. Most of them play like any other pokemon.

I propose giving every legendary a new type exclusive to them. Haven't decided on a proper name yet so I'll just call it the hidden type. The type's gimmick is that it has no set resistances, weaknesses, immunities, etc. Every legendary with the type has their own unique set of resistances, weaknesses, immunities, etc. Just labeled under a single type for simplicities sake. For example, both Lugia and Raikou are hidden type but Lugia will be resistant to water, ice, dark and weak to electric and dragon while Raikou will be immune to electric and ground, resistant to water and flying and weak to fire. All their signature moves will work in a similar fashion. They're all super effective against different types based on the move. For example, spatial rend is super effective against fire, water, grass, ground and flying while being resisted by psychic and non effective against dark.