Apologies if this has been asked for already (I'd find it hard to believe if it hasn't) but I couldn't find it using the search. Here goes:

* Manually link tracks that work well together
allow us to connect 2 tracks that work well together. A small "chain link" icon between 2 decks would do it and link the tracks together (or dragging one onto the other, perhaps with a certain key press). Next time I load one of them, the other(s) appear in a "linked tracks" list.

* Auto-recommend tracks based on rules
in addition to the above, allow us to create rules (similar to smart crates) such as
"show all tracks that have bpm +/- 4% AND have key +/- 3 AND (and other criteria such as genre, tag, comment etc)
see how rekordbox 3.0 has done this now, they've pretty much nailed it. And Traktor for iOS has a simpler implementation (based on bpm and key) which is a good start already.

* Tags
tags would be so much more flexible than requiring us to abuse the grouping or comments fields for organising tracks. even rekordbox 3.0 has tags now.