I've been thinking ways of doing playlist trajectories. It's a tricky problem, though, because the obvious solution is to use the BPM, but that's not a great indicactor of chillness. I think the best way would be to morph between two user-defined "colors" (e.g. start light blue, move to dark red). But this would require (minimal) human intervention. Still, it's quite interesting.
GJay had scaled well on my system (50Gb collection). The only slowness comes at load time when it parses the 5Mb XML file. *Shrug*