Op zich lijkt de tag wel geschikt te zijn, alleen botst het met huidig gebruik:

https://wiki.openstreetmap.org/wiki/Tag:landuse%3Dgreenfield

“The greenfield tag describes land scheduled for new development where there have been no buildings before. A greenfield site is scheduled to turn into a construction site at some point in the future, though the timing may be unclear - sometimes within decades or never. ”