Usually it is extensions - but also keymapping, default utilities, some layout, etc. Basically they structure each specific IDE for what people working in that language would want/need the most.

So, you're kind of going against the grain by trying to make IDEA a general do-all IDE.

This is different than say, Eclipse, where everything and the kitchen sink are under one IDE. This took me a bit of getting used to, but now I kind of like that separation without fear of breaking/worsening one of my "flows" for another lang.