Seeing as a lot of legendaries have either Pressure or some other boring ability, I've tried to come up with unique abilities for them that are inline with their lore and hopefully not broken. Roast me for the abilities and their shitty naming convention or whatever. Or come up with your own.

>Mew: [Name pending]: Functions as a stronger 'Illusion', not only are use disguised as the pokemon in your last slot, you have access to that pokemons moves until the illusion is broken.
(Inspired by mews ability to learn every move and the fact that many researchers claim that sightings of mew are merely an illusion)

>Mewtwo: Terrain Mastery: Depending on the type of move in your first slot, that types terrain is activated. If there is no corresponding terrain, it defaults to psychic terrain.
(Would need access to more fairy moves to be feasible)

>Zapdos: Thundercloud: Sets electric terrain when you use an electric move
>Articuno: Permafrost: Sets hail when you use an ice move
>Moltres: Brilliant flame: Sets sunny day when you use a fire move

>Ho-oh: Phoenix flame: Functions like Healing Wish upon fainting. (or have a 25% heal for all pokemon upon fainting, either one)

>Lugia: Weather guard: Gains a resistance to the corresponding type of weather currently active. (Also gain immunity from chip damage from hail or sand)