Allow multi-dimensional input
For a lot of fitness related goals there are multiple dimensions. Running 5k is one thing, but running it at 9 minute miles is very different from 8 minute miles. In weight lifting, the data are naturally of the form (weight, reps, sets), and an improvement in any dimension represents real growth. Users should be able to report multi-dimensional data and specify a function that maps their data space into one dimension for graphing and tracking purposes.
Fair warning: this one will be a while!
I just thought I'd throw it up there.