Let the user decide which parity/stop bit configuration is used eg. even/odd parity and one stop bit or just two stop bits. [Admin Edit: Already Implemented]
(like the async serial analyzer does)
Support extended 16-bit slave addresses
(Yes, it's not a de facto standard, but it's still widely used in bigger modbus networks. Also it should be easy to implement)
Admin EDIT: Idea - Need to decode both Master/Slave, RX/TX on the same line, to make the MODBUS analyzer useful.