The French campaign group Oui au train de nuit has produced some of the most detailed research about the opportunities for night trains – understandably with the focus on routes within France, and between France and its neighbours.
Its first report from January 2020 (PDF here) sets the scene. There are then two follow up reports – one focused on France (PDF via Google Drive here), and one focused on the European Union (PDF via Google Drive here).
The excellent proposed map of night train services – shown in the header here – comes from the last of these reports, and point “Action 3) Construire un parc de matériel roulant modernisé” (from page 18 onwards) is especially relevant for the Trains for Europe project. The campaigners estimate France alone needs 600 new carriages for night trains, and that the EU as a whole would need upwards of 5000 carriages! While I cannot work out whether these numbers are wholly evidence based, the same sort of work is undoubtedly required for night train routes elsewhere in the EU.