1) Create nested conditional Smart Crates; Smart Crates that contain multiple sets of rules, as opposed to simply multiple rules. See iTunes.

2) Allow regular crates or Smart Crates to be a rule. In other words, create a Smart Crate whose contents are based upon its membership of another crate or Smart Crate (in addition to any other rules you may with to filter on). See iTunes

Sorry to bring up iTunes so much... I do both harmonic mixing and multiple genres and so use iTunes to sort my crates. Since Smart Crates currently get me about halfway there I'd like to see its functionality extended so I can ditch iTunes altogether. It's one less database to deal with.