Metagame name:
Weaknessmons(better name pending)
Metagame premise: Pokemon can only learn moves for
the types that they are weak to, without factoring in abilities.
Examples: Gholdengo is weak to Fire, Ghost, Dark, and Ground moves. Therefore, Gholdengo would only learn Fire, Ghost, Dark, and Ground moves. Existing movepool is ignored.
Example 2: Rotom-Wash is immune to Ground via Levitate, but the Electric/Water typing makes it weak to Grass and Ground. Therefore, Rotom-Wash gets Grass and Ground moves.
Potential bans and threats:

Ghost types get a big boost in this format, as they can use their best STAB options, unlike a lot of other mons.

Normally broken mons like these may be balanced in this metagame, as they lack reliable STAB options and can end up being weaker than the competition.
As for bans, probably the standard OU banlist with these changes:
- Dragapult
+ Chi-Yu
+ Palafin
+ Magearna
+ Annihilape(?)
Questions for the community: Are there any broken moves that should be banned day 1? Is Gholdengo gonna be overwhelming?