Offering people to set tone of voice by actually describing it is a bad idea. Few people have enough imagination and experience to actually imagine and describe adequately a persona they want. Moreover, some people just may be shy to “type that out loud”. Even I was too lazy to describe the persona I wanted properly. So I suggest creating lists of characters with ready descriptions. Maybe with time add an option for character customisation. But the first advice is to at least give a list of traits one can choose from.