Currently, ProSites allows for a number of different limitations to entice users to upgrade to higher tiers (e.g., premium plugins and themes), but the ways in which an individual site can be restricted are somewhat limited (i.e., primarily post/page quotas and upload quotas). As a fairly large number of hosting companies are now providing virtually unlimited data storage (or data storage at such low prices as to make no difference), it seems that another way to restrict sites would be useful. I'd like to propose a ProSites module for limiting sites based on the number of visitors.
In essence, the module, when activated, would create a visits limitation similar to the quota limitation currently offered by another module. e.g., a site could be limited to a thousand users a month instead of 1GB of upload space. This would provide a more realistic and more useful way of limiting sites for those with hosting packages that either do not limit storage space or who pay such a low price per GB that storage is not a concern.
As storage costs continue to drop, it seems that many hosts are transitioning to alternative usage-based pricing (e.g., compute cycles and bandwidth [admittedly, bandwidth charges are nothing new]). Limiting sites based on the number of visits would more closely align with the actual cost of hosting that site than would limits based on data storage.
As for the mechanics of this module, it seems like this method of counting visits (from WP Engine) would probably be a workable starting point: http://wpengine.com/2012/04/visitors/. Granted, the specifics of how to deal with numerous users on one IP address (e.g., university students) and other more complicated scenarios would likely have to be worked out and tweaked over time.
[Hopefully this suggestion will prove more popular than my Glossary plugin suggestion.]