What seems like an easy but useful starting plugin would be one that limits note pitch range. NOT by truncation but by bumping notes up or down an octave at a time until they fall within a prescribed range. This would be a good way to take a series of root postion chords and give them good voice leading characteristics.
To go along with this theme an option to keep the original bass note as the lowest note would help. Should be a switch because it will not always be material that has the bass.
A variation on this theme is a sort of expander. You let the user prescribe the pitch range then spread the notes out to fill the box making sure if there are wider gaps they are between the lower intervals first. To handle cases where the top notes are melody, and thus, should not be changed you should have an option to leave the melody alone and make all adjustments happen to other notes AND keep them below the melody just as you might want the previously mentioned option to keep the bass notes as bass notes. One variation on bass notes would be to keep the bass line intact by moving the whole bass line down one or more octaves to create working space OR to move it up one octave in cases where the processed middle notes result makes this possible.
Basically this would be a MIDI, Intelligent, Compressor, Expander.