Tera Switch 
Premise:
The Pokémon automatically switches in and out of its base typing and its designated Tera typing at the end of each turn (think like Hunger Switch). Consequently, if the next turn passes with the Pokémon in their Tera typing, they will change into their base typing.
Bans:
Clauses: Standard OMs, Sleep Clause Mod, Terastal Clause, Tera Type on Team Preview Clause
Moves: Baton Pass
Items: King's Rock
Pokémon: Annihilape, Chi-Yu, Cyclizar, Flutter Mane, Houndstone, Iron Bundle, Koraidon, Miraidon, Palafin
Abilities: Moody, Shadow Tag, Arena Trap
FAQ/Mechanics Stuff:
When does the type change happen/revert?
Pokémon start the turn in their base typing, and at the end of the turn change into their Tera typing. Consequently, if the next turn passes with the Pokémon in their Tera typing, they will change into their base typing.
E.G. Glimmora w/ Grass as its Tera-type.
Turn 1: Glimmora is a Rock/Poison-type.
Turn 2: Glimmora is now a Grass-type.
Turn 3: Glimmora is a Rock/Poison-type again.
and repeat.
Switching out preserves the type the Pokémon is currently in, similar to regular Tera.
What happens with type changing moves?
Because the type change is an end of turn effect, Soak and Magic Powder would change the Pokémon's type, then they'd immediately lose it to whatever type corresponds to what have to change into (base-type or Tera-typing). Similarly, Reflect Type copies the opposing Pokémon's current type, then gets replaced at the end of the turn. Double Shock follows a similar premise by dropping the typeless part at the end of the turn.
The type of Revelation Dance always matches the user's primary type, so it can change type as the Pokémon's own type changes during the battle.
What about Tera Blast?
Tera Blast would assume a Hunger Switch mechanic, where it is Normal-typed on turns where the Pokémon is its base typing, and whatever type its Tera-type is on turns where it should change to said type.
What about STABs?
STABs would follow a similar logic to Tera, in that Pokémon will have increased STAB output if they Tera into one of their original types, and they keep their regular STABs when they have their Tera type.
E.G. Dragapult with Ghost as its Tera-type would have a 2x bonus to its STAB and keep its 1.5x STAB bonus on Dragon moves whenever it is a mono Ghost-type (its Tera typing), and lose those bonuses when it is in its base typing.
Dragonite with Normal as its Tera-type would have regular 1.5x STAB on Normal-moves on turns where it is mono Normal-type (its Tera typing), and would always keep its regular 1.5x STAB on Dragon and Flying.
What about Libero/Protean?
Pending coding-stuff, Libero/Protean would simply have their regular effect of changing the user's type once, regardless of their current typing.
E.G. Cinderace w/ Fighting as its Tera-type.
Turn 1: Fire-type, uses Gunk Shot, turns Poison-type. Tera Switch means it's Fighting at the end of this turn.
Turn 2: is Fighting-type, Libero does nothing regardless of move clicked (because it already activated.
Turn 3: turns Poison-type.
Council:
Blitz (Leader)
Clastia
Ducky
Kalalokki
in the hills (Co-Leader)
