If we are talking about actual Raids, then they should appear randomly at random difficulties. For example, if they make the PvE mode like Destiny 2 where you start in a Hub and you can just explore the world around you, then there could be a message saying “Raid starting at Paris in 5 minutes”, so all players that want to participate can teleport to Paris or just fly in there if there is a sort of spaceship system.
It’s weird to explain, but those who played Destiny 2 might understand