CD's to DVDs to Blu-Ray progression has been done simply by designing narrower beams, another approach has been "layering", Holographic media is much more advanced using light patterns and inference etc, this is complex.
I think a much simpler approach exists, which can be done withing the current technologies without the need to do complex holographic mechanisms,
optical media tracks are flat and uniform, however if each track was to be curved, then in theory you could 'pack' more surface area in the same size constraint, you could then have a multi angled laser reader/burner to be able to read these 'curved' tracks thus further increased the number of 'tracks'