As a good downhiller, I like the idea of downhill races. It would diversify racing in a direction similar to mountain running, which has both uphill races and up and down races. But I think it should be on a more ridiculous kind of course, so that there is no confusion about what it is. As a side benefit, you don't need world class marathoners to do it, so talk of ruining real marathons by drawing away talent would be moot. It should be lower key, like Big Wave's (Dave Torrence's) downhill mile.
I could see myself having fun in a marathon or half marathon down a road like Haleakala (10,000 ft in 35 miles). I'd prefer the half marathon myself, having never raced well beyond about an 1 hour 45 minutes. I think that even at 47, and never all that fast when I was younger, I could probably go 1:02 in a half marathon down a road like that. There are probably 2:20-2:30 marathoners that could go sub-2:00 marathon on a road like that.