Its technically doable, but it wouldn’t be easy. The problem isn’t in making federated user authentication or servers, its just making a game that is flexable enough with powerful enough modding tools to be worth using compared to just making a game from scratch.

As a simplified example, lets say you want the default mechanics set of your game to be a first person shooter. This means you use 3D graphics, optimized for higher-detail enviroments and include features that would be most useful for that, such as a simple optimized 3D physics system, and tools for making detailed 3D levels.

In this new game, someone wants to make a 2D platformer. What additional tools will they need? A way to lock physics to two dimensions, or possibly even a new physics system entirely, A way to manipulate the camera separate from what the game started with, an orthographic camera, possibly a new lighting system, possibly different optimization techniques, the ability for the modder to replace the player controller and change it’s mechanics entirely, and the tools to use all these to make new levels.

By that point, you’re half way to making a new game engine, and the modder is 90% of the way to just making a standalone game.