Generally I think it is really important to have a import/export function of pretty much everything (parts of maps, templates, prefabs and blueprints of anything like items, created encounters, NPCs, weapons, treasures, all kind of scripts and scripted systems, light templates, visual effects, material and and and).

I am mainly addressing things you can make with the editor out of the box without any custom content. (combined with custom content would be also important)

To have a prospering modding community, there is a need to do small details, prefabs and blueprints for those that dont have the capacity to do a real big complete playable module but just tinkering around with one aspect of the game and providing it (templates, prefabs, blueprints) for the community (via nexus / workshop).

One guy just blueprints of all kind of orcs with nice names, colors and stats and maybe even encounters, the other girl is writing new usable scripts, the next one makes just beautiful plain maps and slow-joe-in-the-back-row just makes stunning waterfall vfx and materials so that finally MR. Not-so-much-time-and-capacity downloads all kinds of usable stuff imports these in his module and focusses on writing the story and dialogs.

Beside the importance for lone-wolves this is a must for any team creating playable content.

This must be easy uncomplicated and stable. Export - Import and voilŕ!!