The current Freezing Level display is Above Ground Level (AGL). As pilots, we need to compare the forecast freezing level to the instrument routes available (e.g. the Minimum Enroute Altitude or MEA), and these are published as relative to Mean Sea Level (MSL).
Here's an example of Freezing Level contours in MSL:
So the enhancement request would be to provide the option of displaying Freezing Level as MSL rather than AGL.