Unity or Unreal Engine?

[Kinsie]

external file system, scripted actors, user mods etc etc

In that case, unless you're willing to hack in all those capabilities yourself, the answer is neither.

[NiGHTMARE]

One of the most moddable modern engines is GameBryo - it's used in Civilization 4 and Elder Scrolls: Oblivion, which both have tonnes of mods. It does have the major disadvantage of not being free, and is apparently somewhat difficult to work with.

[Snarboo]

external file system, scripted actors, user mods etc etc

In that case, unless you're willing to hack in all those capabilities yourself, the answer is neither.

I've seen more Unity games with modding than UE honestly, so it might be worth looking into how Unity devs handle it.

[Kinsie]

external file system, scripted actors, user mods etc etc

In that case, unless you're willing to hack in all those capabilities yourself, the answer is neither.

I've seen more Unity games with modding than UE honestly, so it might be worth looking into how Unity devs handle it.

It's possible, but it's not an engine-native feature, you'll have to roll your own solution.

Unreal Engine 4 has some things that are kinda sorta mod support-y, but they're either hilariously unfinished or not properly exposed to users. In addition, you're not even allowed to pack the editor in with the game - you have to make it available to Unreal Engine subscribers via GitHub.