Minecraft wiki entry for Thunderstorm_survival, associated text: Every single lightning strike could potentially spawn a skeleton trap. (See below).
Creepers will become charged if struck by lightning. This doubles their blast radius and damage. You can tell from the blue aurora around them, these are rare spawn however it is good to have some blast protection armor or a shield.
Pigs will turn into zombified piglins if struck by lightning. These are a version of piglins normally found only in the nether and are passive until hit. However this is a rare spawn.
Villagers will turn into witches if struck by lightning. These mobs throw potions at the player and drink health potions themselves.
Phantoms will spawn high in the sky and dive at you in depleting health. Usually you don't notice them so keep an eye on the sky for spooky birds.