I'm also strongly in favour of adding the whole lowlands to this map from EBFN Koksijde AB to EHLW Leeuwarden AB !
For Flanders there are historical air photos available for the public that are very usefull for further developing and creating this area,
full detail photos are available for 1971 (Black & White): https://www.geopunt.be/?service=https%3A%2F%2Fgeo.api.vlaanderen.be%2FOKZ%2Fwmts%3Flayers%3Dokzpan71vl
and for 1979-1990 (color): https://www.geopunt.be/?service=https%3A%2F%2Fgeo.api.vlaanderen.be%2FOKZ%2Fwmts%3Flayers%3Dokzrgb79_90vl
I do not know any equivalent for Wallonia and The Netherlands, maybe someone can look into this?
These links can also be usefull for the currently planned phase 3 EBBL Kleine-Brogel AB, I recently been inside Kleine-Brogel.