First of all: thank you for offering this great service for free. It is more than our hole national weather service in Austria can provide!
To close the gap between the “classical” representation of model data and your site I think a 2nd layer, which visualizes the numerical data as a contour plot, would be a general solution. With an independent z layer every user can combine 2 arbitrary model values (for example 850hpa temperature as colours and the sea level pressure as contour lines).
A second improvement would be the possibility to disable the color gradients. So it’s easier to detect cold fronts on the 850hpa temp map.
Thanks and keep on your great work!