Very good plan!
I am also willing to completly rewrite the rule system. Instead of anonymous / logged in / Sysops / Bureaucrate, I am willing to assign users to groups, each group having rights like "edit, move, special page, site administration, mediawiki, protecting page ...".
My question here is: How do you plan to manage the target pages?
Do you want to assign permissions of a group to each page? I think putting pages into groups is more practical than treating them individually.
There are already the concept of category in wikipedia. We may use category name as the natural group name for a page and assign user group's permissions to the page's category rather than the page itself.
I surveyed and tested other wiki software these days and found Tikiwiki may be the best for user permission management. eGroupware is good also for this. Some of them even have interfaces to access LDAP sever. You may refer their work to design and implement the right one for wikipedia.
Another thing I want to mention is Mambo Server at mamboforge.net. I almost get addicted to trying it those days. It is something like an web based operating system and you can install and uninstall components(poll, forum, pic gallery, doc management, online-shop) conveniently. Some people there are also trying to port wiki into this flexible framework.
Anyway, keep our eyes open is good for wikipedia for a long run.