It is not possible to determine a persons ethnicity through the API. It is possible to see the language the broadcaster has set, so you could potentially do it based on which users have ‘da’ set as their language, but this would have some margin of error as there could be non-danish people who are trying to speak that language to learn it, and there could be Danish people who are speaking English to appeal to a wider audience.
While creating a map of followers and the connections is certainly possible, there are some things you wont be able to easily do, such as knowing what games a user follows as to my knowledge both the old v5 endpoint and the new Helix endpoint only return a list of users that are being followed, and doesn’t include games.
You can get some data on Communities through the API too which could provide some additional data on how users are connected, but currently the API has no way to get anything relating to Tags.
Also, as mentioned in the Developer Agreement you may need to contact Twitch for a separate agreement to do what you intend to do, as it sounds like this use case would fall under ‘Research’.