Right now the wire for the bone controllers or bones in general is something like one pixel thick, and that added to the ungorgeous colour selection presets for the bone groups, makes viewing / selecting controllers on a rig painful.
So my belief is this can be tackled from two different areas, first to manually improve your rigs, and secondly for compatibility with previous rigs.
in the "display category" of the bone properties, below the "wire frame" tick ther could be a wire frame thickness that would handle the wire thickness display for that bone alone, this would allow a rigger to make some controls thicker than others instead of making geometry extruded and such, also giving the animator an easy access if he wants to make some of the controls he uses the most more visible.
on the other hand,
in user preferences > themes > 3d view:
an option that would tackle specifically all the shapes for bones in pose mode, I believe it isn't really necessary for bones that don't have shapes.
As an added note, being able to override display method for different bones would also be nice, show some bones in octahedral, others in stick, some in b-bone , etc would also be wonderful for building better interfaces, and this could be also in the Bone Display under Bone Properties in the Properties panel.
I don't have images for this, but i think the ideas, and solutions are pretty straight forward, and would represent improvement in visualisation and handling of rigs.