Ah... Ok, I get what you mean. I think any of the open source CMS packages (Drupal, Joomla, Wordpress, etc...) can be configured to do this particularly those that are integrated with jQuery (or one of the other "ajaxey" libraries) but it will be up to you to configure how much or how little you are going to expose this to your audience. I find from an administrative point of view, pretty much everything is drag-n-drop these days but on the user side because you tend to permit less administrative actions, they will have less of those abilities.
You may have done this but if this were my project, I would spend a bit of time in project manager mode to chart out all of the activities I want available for the whole project and then I would a list of 'roles' like Master Admin, Admin, Site Owner, Editor, etc... and chart out what these individuals would have access to so that I could figure out in definitive terms what all of the activities are and who can access them and how detailed or granular their abilities would be.
I would also figure out how scalable I would need the site to be. Some CMS packages are very simple to set up and launch but when you want to scale them right up you run into limitations so you definitely need to balance ease of development against scalability, dependability and security.
I think what you as the developer or project manager will need to do is look at the project in broad strokes as find out which multisite CMS will provide you with:
1) Scalability to easily spawn x number of site where x is the maximum number of sites you anticipate in spawning
2) Provides you a user management system interface to create the number of user roles you need with adjustable permissions
3) A dependable, secure and well documented code base with a security team to keep on top of and release timely security patches. Nine times out of ten, you want to be able to extend the CMS to achieve your goals and if the CMS has a rough API or poorly documented API, you could run into trouble extending it.
4) A knowledge base, forum, IRC, etc... where you can search for answers and post questions
I don't think there is one particular open source package that does that perfectly so you'll have to assess and make up your mind on which one does the best job and then commit yourself to either learning how to achieve your goal or find developers to work on the project for you.
In my mind this is a big project. There's an 80/20 rule that comes to mind when thinking about it:
It will probably take 20% of your time to get 80% of the functionality to work and 80% of your time to get the last (but most important) 20% of fine tuning and tweaking.