Tbh I don't think I've seen the 1800+ crowd so united behind a ladder idea before, if the 1800+ pool was partly or wholly mapgen for a month I think it would go down very well.
Guess it's not a big deal either way, just might see all the tryhards in the "casual" queue which will be a laugh but people get what they want, that's what matters.
edit - guess the risk is splitting 1v1 player base, especially if the "casual" queue isn't pure mapgen. If it's pure mapgen then the hosting of custom mapgen 1v1s will just move to an automatch feature, which would be an improvement, but if it's not then it might get a bit weird in a three-way split between the two queues and custom games.