Well the "ten days" are now up
I usually think that splitting plan quotas into 3 periods, as has been suggested in the past, would make plans too complicated. However, after spending a few more minutes thinking about it tonight I think I would prefer three periods over the off-peak extension proposed. My reasoning is that, especially over weekends, opening up the period between noon and 6pm to a large (currently 180GB) quota might be detrimental. Instead, how about moving off-peak to 3am to 9am, then have peak1 from 9am to 6pm and peak2 from 6pm to 3am. Then you can essentially just double peak quotas (copy the current peak quotas into peak1 and peak2, turning the 10GB+180GB plan into a 20GB(10GB+10GB)+180GB plan for example) instead of up to a possible 180GB between noon and 6pm.
You'll just have to keep peak2 up to scratch against other ISP's peak quotas, as you have been doing with your normal peak period. Then just keep peak1 the same as peak2 for symmetrical simplicity.
There are cons to almost any idea, but I just thought I would put mine out there. Though I guess it is slightly similar to dbr's idea, except without the customisation ability which would add more complexity and make it even harder to put up solid comparisons against 1 and 2 period plans from other ISPs - at least in my opinion.
Edit: Alternatively, do as you are suggesting (make noon to 6pm off-peak) but make it chew through the quota at 150% or 200% the normal rate. That way it is still a boost for most people, but the heavy downloaders will be kept to the real