I am suggesting to add Isobar and Isotherm Contours for upper-air charts: 850hPa, 700hPa, 500hPa, and 250hPa.
@bunhyan Hello, these isolines are already available (Temp isolines and Geopotential isolines) in the side menu.
As @Korina said, you can find Isotherms and Geopotential heights in More layers…
Isobars are never shown at high level, afaik. Geopotential heights are the altitude of a pressure level e.g. 500hPa. They are usually expressed in dam (decameters) but in Windy they are shown in meters.
Here is an example of Isotherms at 850hPa.