While it's probably no use to rename the
/etc/shadow files, if you want added security you might want to look at PAM (pluggable authentication modules) and NSS (Name Service Switch). Like here.
PAM can be used to add authentication modules that, instead reading their authentication ifnormation from the standard files, read it from an other source, like from ldap or a database. Using it would mean the
/etc/shadow can be almost completley eliminated.
NSS complements PAM by making some of the name resolution (like which groups does this user belongs to) independent from the standard files (
/etc/groups). Using it would mean your passwd file will potentially only contain a fallback option for root, and nothing more. Using SSH keys to validate root login would also eliminate the need to have a root password inside the shadow file (although it might be desired if the SSH connection breaks).
Alternatively if you don't want to authenticate your users via a separate database or ldap host, you can also create your own PAM and NSS modules, which read their data from a non-standard file, although I wouldn't recommend this option.
When you want to try to use them never forget to keep some kind of fallback to a known, working authentication layer, otherwise you can lock yourself out of the system, even with root.
Note that not all applications support PAM (a lot of them do however). NSS however can be used to implement authentication for apps that don't support PAM, and some sites I've read about NSS actually suggest this approach. This however means that the NSS module will supply the (potentially) hashed password to anyone who can access the NSS authentication layer, which is almost always something you want to avoid (it's basically the same as giving non-root read access to the shadow file)! So if you're going this approach always make sure that NSS is only used to provide the user with the basic data (like the contents of
/etc/passwd), and PAM is used as the authentication layer.