This is just an idea. It's okay if this gets rejected. But in addition to being able to assign and unassign permissions to and from usergroups via the respective "Assigned Permissions" and "Unassigned Permissions" tabs in Special:ManageWiki/permissions, it would be nice if there was an additional tab that allowed users with the (managewiki) permission to be able to revoke permissions from specific usergroups.
Just to be clear in case you don't understand what this would do, revoking permissions from certain usergroups would cause users in those usergroups to be unable to access those permissions, regardless if they are part of any usergroups that would otherwise grant them access. Revoking permissions from usergroups would cause those permissions to appear crossed out in Special:ListGroupRights.
Out of all the permissions that are currently assignable via Special:ManageWiki/permissions, there would obviously need to be a restriction regarding what permissions can and can't be revoked from certain groups. Permissions like (read), (managewiki), or anything that a certain usergroup should have access to no matter what would obviously be among those permissions that shouldn't be revocable. And obviously the blacklisted permissions wouldn't be revocable. Though there aren't many non-blacklisted permissions that I can think of that shouldn't be revocable.
The reason I'm requesting having an option to revoke permissions from certain usergroups is so that if you want to revoke a certain permission from certain user but don't want to revoke the entire usergroups that the user is part of, you'd be able to keep them in the usergroups that you want them to be part of. It would also be useful for testing what permissions are dependent on certain other permissions.