You can input whatever you wish at the prompt as long as it's a valid ML expression.
>> input('?')
?[2, pi, sqrt(10)]
ans =
2.0000 3.1416 3.1623
>>
Of course, you need to give your users a clue as to what it is you're expecting from them! :)
And also be prepared to handle whatever it is they give you.
There are other input options besides just input to give more control over the interaction.
Look at inputdlg for example.
