FeatureUnion currently passes identical data to each constituent transformer. Often one wants to differentiate between groups of features in how they are transformed. While this is possible by making each stacked transformer a Pipeline consisting of a pre-determined feature selector and another transformer, this is cumbersome.

A parameter should be added to specify which features are routed to which constituents. This is not necessarily trivial to design, particularly because the input X to FeatureUnion.transform need not be a conventional 2d feature array (it may be a list/array of dicts, texts, or other objects).