Currently, as shown by the available defined tags, there is no way to make it so while possessing a mob, you can use your main and offhand, hotbar, and possibly armor slots, but not the rest of the inventory. To fix this, I suggest adding in a tag, such as #requiem:inventory/hotbar_only or something similar. When an entity is in this tag, anyone possessing that entity can only use their main/offhand and hotbar slots, but the rest of their inventory would be blocked. If that entity also exists in #requiem:inventory/armor_users, the entity can use their armor slots as well.

I have a couple of ideas for the default contents, which could either be:

• Empty, preserving all default behavior from previous versions but still allowing customization as shown above
• Contains minecraft:villager and minecraft:piglin as both entities only have 8 hidden inventory slots when not being possessed, and limiting to hotbar only would be closest to that behavior.

Fixed in 2.0.0-alpha.10 ☑

Sure, why not. We don't want to make villagers and piglins too annoying to play though, so first option sounds more likely.