Several people have already suggested parameters, and even conditionals based on those parameters. Those ideas are great, as well as the idea of context-free expressions to help generate relative values. The expression that I REALLY want is RAND(a, b). This would save so much code when it comes to generating random values in a range.
Seeing as a lot of us have been using multiple rules with minute changes all this time to accomplish the same thing.
I think the random function should have three arguments though, the min value, the max value and a step value. So that way you can generate a random number between 1 and 2, yet confine the result so it only appears as multiples of 0.10; i.e. 1.10, 1.20, 1.30 etc.