Hierarchical feature types
Currently, there are only 4 bottom-level feature types: interval, ratio, nominal, and ordinal. However, these are a hierarchy, where interval and ratio are mutually exclusive subsets of "numerical", and nominal and ordinal are mutually exclusive subsets of "categorical". These should all be subsets of "all".
There should be an easy way to add new features to the feature set, and when they are added, it should automatically recognize all of the higher-level feature sets that they belong to.