So far there have been suggestions for Italy and Romania.
After a scouting trip to one of the Italian locations we got told that it is not available.
another location proposal in italy is the valley at Malga/ Casera Chiampis:
Malga Chiampis is a bivac hut. i don't suggest to have the gathering in the hut, but in the huge valley floor around it. in 2018 a thousand rainbow people gathered there, it's a huge open area at 1200m ASL, surrounded by peaks just shy of 2000m ASL. the place is about 5hours hike from the road, hiking trail begins close to Tramonti di Sopra. there should be drinking water, else the rainbow gathering would not have been possible.