Logically, selection of certain gods, backgrounds, or such could "unlock" special tattoos keyed to that selection - and could even require the tattoos be placed in a "concealed" part of the body under certain circumstances - and revealing the tattoo as part of a conversation could even be added in for flavor. Some tattoo's, such as those for a specific thieves guild or organization such as the Harpers could even be obtained during the course of the game, assuming a character could join one. These specialized tattoo's would often not be worn openly - the only time you would see them is when shown to someone as identification (or during "private" liaisons).
Again, fun RP ideas to my way of thinking , but while I enjoy these types of RP flavoring enhancements, not everyone finds them to their taste. And of course, would Larian find it to be a worthwhile use of developmental resources.
