[LelouBil]

Honestly, it seems like the way to go to have a very moddable game is a language that's easily decompiled and modified at runtime like Java or C# (or just JVM and .NET CLR if you prefer).

When they do internal modifications, but also have modders in mind, you get to have an amazing modding API basically "for free".

[account42]

The only thing you need to have a very modded game is a large enough user base. A little bit of binary patching and dll injection never stopped a dedicated modder.