Currently when creating a custom dashboard, that dashboards becomes available on all websites in that Matomo instance for that user (Or multiple users if copied to other users). This can sometimes be an issue for Custom Dashboards that have widgets that only exist for specific websites (For example Custom Reports) or widgets that won't have any data (For example Ecommerce widgets if the website is not an Ecommerce site)
One way to overcome this issue would be to allow users to set specific dashboards to only be accessible for specific Site IDs.
Maybe an additional feature would be to allow a custom dashboard to be set as default for specific Site IDs so that a user doesn't need to change between dashboards depending on the site they are viewing.
Another god idea would be to share the dashboard to several user in one click (instead of having to share 100 times for 100 different users... 😕)