I'll answer this question the other way 'round, which is what *not* to buy. And the top of my list there is the Canon 50mm f/2.5 Macro. From my perspective it is wrong every way around. For working with critters, there is no working distance. It only focuses to half life size (1:2). You have to buy a converter to focus to life size, but this changes the focal length to ~75mm. This lens uses the old AFD focus system, which is ridiculously slow today. At f/2.5, you can't produce a truly shallow depth of field for landscapes or portraits. Plusses are the sharpness and 52mm filter size.
This lens should have been replaced fifteen years ago, IMHO. To make something which sells, I think Canon should make a "value added" replacement, something that would sell even though 50mm macros are not all that popular. I can see a couple ways to do this. One would be to make a 1:1 macro lens with f/1.4 maximum aperture (very difficult to engineer, but then that would keep competitors from creating a competitive product and it would have a high price and profit margin). Another would be to create an f/2 macro which focuses to 2:1 or twice life size. I personally would be thrilled with either, especially if the lens had USM rear focusing and a 58mm filter thread. A third possibility is creating a tilt-shift macro that focuses 1:1.
Because of the popular 65mm MP-E, and folks using the 90mm tilt-shift with extension for closer focus, I suspect the first idea is the most marketable. This is just my opinion, and I would be interested in hearing what others think. Would you buy one of these lens ideas if it came on the market? Is there another "value added" design that would entice you to buy a 50mm macro lens? What do you think would be successful on the market?