Which external data can be load at runtime

we start a discuss in our team about external data loading at runtime.

So, right now im try to explain our question in detail.

for example we have a game, and seek find a possibility to load some external data.

By default we use a FBX, but this is proprietary file format. And currently one existing way to load, - is a UE.

If we want to load external 3d files, how easy it will be do. Or possible\legally is it?
Especially about direct file loading without asset, from the folder.

We pursue a goal - modding friendly game, with the “open”, atleast one part of the structure, like 3d models and .txt files with the params(about model GUID and text data).

This is our first project, and probably this is too difficult a task for us right now, but we still wants to know about this more. For future projects.

My english is not perfect, but i hope, that correctly explain the situation and my issue.

-Thank you.