I'm currently using Unity for my main project and I've toyed around with the UDK and Spring, but I have another project I'm thinking of that has some different requirements:
1 - Must be a 3D engine of course,
2 - Must be capable of networked multiplayer,
3 - Doesn't pack the game assets (models, textures, audio, etc) into proprietarily formatted packs, and instead leaves everything open and uses standard formats, thus making it easy for users to create new content and customize the game.
As I mentioned, I'm using Unity for my current project, and I like it, but it wouldn't be very suitable for my next idea - all the assets and scenes in the game are packed very tightly into bundles and mostly converted to proprietary formats, making it difficult to nearly impossible for users to mod the game. It is possible to script your own runtime model importers and such (I think KSP does this, and there's also
this thing available for purchase), but it's not very flexible and creating the sort of functionality I'm looking for would just lead to me creating an engine inside an engine.
I've looked around at other engines a bit, and UDK is nice and all, but as far as I can tell it's in the same boat as Unity when it comes to modding support. Spring is closer to what I need when it comes to asset loading and such, but it's also pretty strictly an RTS engine and wouldn't work for this particular project.
I don't need an engine with top-of-the-line graphics capabilities, I just need something flexible and open for modding that's also capable of networking. Can anybody recommend something?