We don’t have plans for a symmetry ruler actually. Could you please elaborate a bit on why it would be important to you?
You can actually already just load images to use for reference. This could be a model sheet, backgrounds, layouts, perspective grids/lines, photos or even video. In the current version video must be imported as a sequence of png or jpg frames or an animated gif. This is good for video reference, rendered 3D animation that you need to fit your 2D animation to, rotoscoping and more.
When you load one or more image files (png/jpg/gif) they will automatically show up in a new “reference layer”. Ref-layers can not be drawn upon, but you can adjust timing for it, i.e. add clones, copy/paste, link, etc.
You are perhaps thinking of a separate window to hold your reference in, but having it as a layer is great for many reasons, like easily adjusting its opacity.