You did say that that the feature list was not yet closed, therefore I offer suggestions for two features that would give a workable IP-Centrex / Multi tennant system when combined with A2Billing.
1. The ability to select a trunk based on extension, therefore extensions tagged can use certain trunks/Outbound Routes, but not others - In reality,we can do this in A2B, so a nice to have rather than a necessay.
2. More importantly, being able to stop one group of extensions being able to call another group of extensions. E.g. so that say when a call is parked, or transfered, it cannot be picked or transferred to another group of extensions.
This used to be possible in earlier versions of FPBX (pre-parking lot) by changing the context that the Extensions lived in. E.g. extensions could only call other extensions in their own context.