Override permissions per user I have the following problem and I am currently looking for the best approach to solve it:

I have a site with various entities. These entities all have their own permissions to view, edit, delete and so on.
Now I don't want to restrict the access to an entity per entity type but per entity element, with every user probably having access to different entity elements.

This means that I will need a very granular way to change permissions.

Thats why I would suggest to add a few basic roles to which you can assign your users.
On top of these roles you are able to overwrite the permissions per user per entity element. This would probably happen in a seperate tab in the profile of a user.

If you have any idea on how to do so or you have another approach on my problem I would welcome your response.

Thanks in advance.
I think you can use Access Control Lists. It does not have a UI on its own but your can use probably content access to give you the ACL functionality.  

NOTE: After installing and configuring the ACL is not part of the user profile but part of the node (as you have suggested in your question)