Help with Lair/Legendary actions

[Calen]

Hello, I am working scaling up a hydra and a dragon turtle as (separate) boss monsters for some possible level 20 boss encounters.
I am trying to think of some interesting legendary actions or lair actions that these creatures could take. Any suggestions?
These creatures would be encountered alone or mostly alone in a vaguely swamp/jungle/marsh environment.
Thanks for your input.

[Amnestic]

Lair action: Swamp gas! Create a bunch of swamp gas clouds (size varies depending on the size of the battlefield) that are opaque and poisonous to move through. They stick around unless dispersed by wind, creating more and more the longer the fight goes on. Boss is immune, obviously.

Lair action: Whirlpool! Creates a whirlpool that sucks up creatures in an area and brings them together, deals bludgeoning damage to them when they slam into each other.

Lair action: Stampede! The boss monster roars and it triggers a stampede of creatures across the battlefield. Creatures caught in a wide line that stretches the battlefield need to make a strength save or get trampled prone and take some damage.

[Unoriginal]

Legendary Action: Wings of the First World: Calling upon their draconic origins, the creature grows a pair of dragon-like wings for a brief moment, allowing them to fly up to 40ft.

Legendary Action: Breath of the Gods: Calling upon their draconic origins, the creature does a breath attack dealings cold, fire, acid, poison or radiant damage.

Legendary Action: Law of the Jungle: the creature attempts a bite attack on an allied or non-hostile being. If successful, the creature regains HPs equals to half of the inflicted damage, rounded down.

Lair Action: King of the Jungle: the creature bellows a mighty roar, causing XdY Beast-type beings of CR Z or less to come in the Lair.

Lair Action: Uprooting for the Enemy: the creature causes a tree to fall on its foes. Anyone in the zone must succeed a DEX or STR save to avoid taking damage and get stuck under the tree.