The 180mm focal length was always a bit of an odd-ball. Also the 180L macro has about the slowest AF of any L lens I've ever seen (certainly because of the huge glass elements involved, but even behemoths like the 85mm f/1.2L are faster).
A 200mm f/3.5L macro with hybrid-IS and fast(er) AF would be a welcome addition to the lineup.
But really I'd love to see them add IS to the 135 f/2L & 200 f/2.8L, or really any other for-mere-mortals fast telephoto prime for that matter.
I own the 180L and have never understood the complaints against it, other than the weight. The focal length is ideal for macro and I have used AF on it when I calibrated it with FoCal, but I'm not sure I've ever used it in the field. For any other use, it's a terrible lens, but for macro, it's perfect. From what I've read, the AF is slow for precision, and if you set the focus limiter to the far distance, it's actually as snappy as nearly any other USM lens.
I think it's stupid to add IS because the DOF is so shallow (even at f/16) that IS would be all but useless at 1:2 to 1:1 macro distances, and it's a poor choice for just about any other use compared to Canon's other offerings. I'd much rather have f/2.8 and 9 rounded aperture blades like the 100L macro.