I would like to see a formula question where the variable values can be generated by a formula, and does not necessarily have to be numeric. Likewise, answers do not necessarily have to be numeric. As an example take a question where a student is being asked to convert a hexadecimal number to binary. In this case "convert hexadecimal `x` to binary". In this case I need to be able to write a formula to create possible values for `x`, and that formula needs to be able to generate non-numeric string values like "3BF" or "9A7".
Likewise if I asked a student to 'convert binary `y` to hexadecimal", I need to be able to write a formula which will generate a random string of 0s and 1s as possible values for `y` (such as 00110), and I also need the student to be able to enter a non-numeric answer, such as "3BF" or "9A7".