Timur, how ergonomic is a box going to be? I think its self evident that the box isn't ergonomic by itself.
That and the need for ND filters just cries out for rails and a matte box. On top of that there is the lack of any way to put it on your shoulder and the need for a forward mounted LCD and I can't think of any reason to take it off of rails other than to use once in awhile as a stealth cam.
Pretty much the handles, shoulder mounts, etc will be aftermarket. I think the FS100 is going to be a camera that you mount on rails and leave it there.