It would be interesting to add a right panel in the track library suggesting a next track to play. The selection proposed could be 1) automated through specific selection rules (similar to smart crates) or 2) "hard-linked" through meta-data associated to each track.

1) In the first case (~smart suggestions), example of filters could be:
-> track is within +-5% of BPM
-> track is a specific key or in key interval (e.g. +-1 on Camelot Wheel)
-> be based on gig history (If I often played A before B, B would be suggested when A is playing)
-> matching keyword in comments
-> ...

2) In the second case, each track would have a meta-data field "Complementary Tracks" (or whatever you want to name it!) that could hold a limited number (e.g. 5-10) of tracks, specified by a file path (there would be issues when moving a library or changing track paths...). I'm not sure how and if Serato stores meta-data beyond usual mp3 containers and if this idea can be implemented.

Of course, the "currently playing track" would have to be selected through a deck selection (1,2,3 or 4?) so that the correct "complementary tracks" can be displayed.

This feature could be used to mimic the "remix set" by linking multiple layers (vocals, fx, hats, kicks, ...) of a given track together.