Recreated the thread since the original was kinda "hijacked" with random (and most arent even true) comments about Forge, Fabric and modding in general, plus i felt the need to try "clarifying" what i want in first place.
This is "basically" a way for modpackers to load datapacks in every world from a common folder in the root of the profile directory. "datapacks" would be good. Basically in a similar way of the vanilla Resource Pack system (which loads "common" packs from "resourcepacks" folder.
Plus a new option in menu to manage this common datapacks, identical to the Resource Pack menu (shown below); the user can enable or disable global datapacks when he wants. Additionally in a similar way of how vanilla have a "meta slot" fixed at the top of the list (since the local datapacks will always get priority than global ones, just like vanilla resource packs does) representing the original assets, named "Default - the default data for Minecraft", a "meta slot" would be added to represent all per-world (aka the ones normally loaded by vanilla) datapacks to be loaded for each world. They should continue to be managed using the /datapacks command.
Screenshot of the resource pack menu for understanding: