It would be nice to have a prediction of when it's going to start and stop raining, in the form of a map. In the Netherlands the website "buienradar.nl" is often used for this, but it doesn't let you choose the data sources, and isn't nearly as aesthetically pleasing as windy in my opinion. The feature looks similar to the weather radar feature in Windy, except it also shows the predicted continuation.
In Windy the predicted amount of rainfall is available in tabular form for one specific location, but it would be much nicer if this information could be scrolled through like the weather radar map. There's also the rain accumulation feature, but that tends to be too vague to get a solid idea of when the rain is expected. The weather radar feature gives an idea of the motion of the rain clouds, but it is currently left up to the user's imagination how the motion will continue, and tends to reset to the start exactly when you're at the part of interest (i.e.: the most recent data).
Both a map based visualization of the forecast, and an extrapolation of the radar data would be interesting to me.