Chat/Instant Messaging between staff.
Add ability to perform basic calculations on custom fields.
Calendar functionality for tickets/scheduling.
Resource scheduling.
Cc.
Ticket status with color changing.
Specific help topics require manager approval before ticket can be closed.
Define block of purchased support hours by client/company. Have billable hours field in ticket that subtracts from block when closed.
Report creating an invoice showing billable hours.
Merge or parent ticket functionality.
Client login with ability to "brand" client interface to particular clients (client logo, etc.)