Whether a driver should be a Unicode driver or an ANSI driver depends entirely on the nature of the data source. If the data source supports Unicode data, the driver should be a Unicode driver. If the data source only supports ANSI data, the driver should remain an ANSI driver.
A Unicode driver must export SQLConnectW in order to be recognized as a Unicode driver by the Driver Manager.
A Unicode driver must accept Unicode functions (with a suffix of "W") and store Unicode data. It can also accept ANSI functions, but is not required to. (The Driver Manager does not pass an ANSI function call with the "A" suffix to the driver, but converts it to an ANSI function call without the suffix, then passes it to the driver.)
A Unicode driver must be able to return result sets in either Unicode or ANSI, depending on the application's binding. If an application binds to SQL_C_CHAR, then the Unicode driver must convert SQL_WCHAR data to SQL_CHAR. Note that the driver manager will map SQL_C_WCHAR to SQL_C_CHAR for ANSI drivers, but does no mapping for Unicode drivers.