Here's an off-the-wall algorithm idea: a pitch shifter that duplicates Arvo Part's tintinnabulation technique. The basic idea is that you harmonize a melody note with a tone from a specified triad, with no unisons or octaves. The available "positions" for the harmony voice are one or two triad notes above or below the melody note. For example, if you specified your triad as E minor and the voice as -1, then playing a D or E would generate a B note in the harmony, while playing an F# or G would generate an E. Ideally, you'd be able to select not only the position for the harmony voice, but also whether it was voiced "close" (the nearest specified note to the melody note) or "spread" (an octave up or down from the "close" note). Controls could be:
-Wet/dry mix
-Voice 1/2 mix
-Triad root note
-Triad type (maj or minor at least, with diminished and augmented if you're feeling saucy)
-Harmony 1 position (-2, -1, off, +1, +2)
-Harmony 1 close/spread
-Harmony 2 position
-Harmony 2 close/spread
…with maybe some filtering controls or independent pre-delay for each voice to use the remaining spots. It would also work as a single-voice harmonizer with delay and modulation controls. I've been messing around with this technique manually, and I think it could be the next killer app for ambient players.
