I have an idea for a MU plugin but unfortunately don't have the skills to write it myself.
Anyway the plugin will be for subscriptions and will hook (if possible) into the Site Admin > Blogs menu pages.
The plugin will be used for different levels of service provided by your Wordpress MU hosting.
I see it as a drop down list with subscription levels you define.
i.e.
Default (standard offerings)
Level 1 (more themes perhaps ?)
Level 2 (more themes and maybe more plugins ?)
A simple drop down form would suffice and we would manually edit a blog if and when they decided to take up a certain subscription.
So a member signs up and gets basic blog capabilities, some themes and a few plugins. However if he wants advanced usage of plugins (i.e. more of them) and more themes he opts for Level 1 subscription and pays you a nominal monthly fee.
To not get into paypal API programming etc at this stage (although the plugin would have the framework for it at a latter stage i guess), Admins would manually change the state of that users blog when the needs arise.
Then in every theme and template that pertains to a level above default we would just insert a simple check to see if this member has rights to run advanced themes or plugins, pertaining to his level.
It would require editing the plugins or themes you want to remain for paying customers but that shouldn't be any more of an issue of something like
if (!subscription(X))
return
At the top of the plugin or theme.
where X is the level of subscription you set.
Providing the check across templates as well would allow you to define advertising for default for instance, no advertising for Level 1 etc etc.
In fact the possibilities are endless.
Feasible ? Do-able ? Anyone willing to give it a go ?