Users that have the admin privs should not be always using an account with admin rights. There should be for them, and only for them, some kind of "su" possibility, with a password to type (or something better) to get enter a super user mode.

Why so? Are you logged in as root on your machine? Same problem, same solution.

This should be however should be configurable. Some sites may have a paid staff of persons that are only admins, not doing anything else, and so log in only to do admin stuff. So we should allow some kind of "root access".

Maybe it should take place in user management of the sys project: instead of admin or not admin, there would be "always admin", "admin", "not"...