Hone in is always incorrect, if you think about it. As you mentioned, you can sharpen or hone your focus on an idea. But then the object is your focus, not the idea. So you still wouldn't sharpen in your focus on an idea. If you don't like the sound of home in, just say hone. Hone an idea, instead of hone in on an idea. If you want to hone your focus, you can do that to, but if you have to add an in after hone, then what you really mean is home in.