All I need help with is creating a new rewrite rule to put all through the router with the exception of one directory, for example named "wordpress."
Based on that criteria, you should add the following rule to the mix:
RewriteCond %{REQUEST_URI} !^/wordpress
Which will make the whole ruleset now be as follows:
RewriteEngine on
RewriteCond %{REQUEST_URI} !^/wordpress
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteRule ^.*$ /router.php [NC,QSA,L]