The serial terminal could have a check box that, when enabled, sends the keystrokes as you type them instead of having to type and press Enter.
The Enter key would act as configured (Newline, Carriage Return, Both NL & CR). except that "No line ending" would have to be one of the 3.
If possible, there could also be a separate text box for local echo, or use the existing text box used for typing. Don´t forget destructive backspace...
If you want a real terminal emulator then why not use one ? I suggest that there are more pressing needs to addressed that tarting up the Serial monitor.