Building further on Custom Fields, presently implemented on a global basis, would be the ability to define Custom Fields for specific Organizations, Groups, ... It gets quite un-Zen like to have non-applicable custom fields clutter up the place.
The tag based implementation of Custom Fields integrates well with Trigger Rules to enable some useful mappings between Custom Fields and Ticket Fields e.g Critical [Custom] maps to Urgent [Priority]. This lets users communicate more that just a description of the request to include metadata.
A step further, is to make Custom Fields specific to scenarios. This would enable implementation of SLA's where customer Organizations use particular terminology not applicable to all Organizations.
I guess there are two aspects for implementing this: (1) Global Custom Fields, & (2) Context Specific Custom Fields.