Extending the range of temperature and wind to the stratosphere as GFS allows, to see the evolution of the stratospheric polar vortex, would be a good idea. But it seems that it would be necessary first to add a polar projection map because the current Mercator projection used by Windy is not really suitable for a representation of this vortex. Or to go back to the 3D view as it was before it was deleted.
Concerning the extension of the GFS forecast range from 10 to 15 days, for me it shows a low interest as most of Windy users are interested in wind, rain and other elements of the sensitive weather. People would be very disappointed by the low reliability of these forecasts. They have an interest for specialists who seek to know the position of centers of action and the trend on a synoptic scale, with ensemble models rather than deterministic models as those used by Windy.