Show national heritage sites ("monuments historiques") on the map
The idea would be to mark in some way (a special symbol?) either "monuments historiques" themselves, or plots that contain "monuments historiques". If possible, the type of "monument historique" classification should be shown (apparently there are different types). The idea is that the destruction of (some? all?) buildings classified as "monuments historiques" is forbidden by law, which can be interesting for squatting purposes - if the building cannot be destroyed, that's one less way/justification for eviction.
The first step would be to search for open data available on the topic.