Hi Marc,
One thing that struck me was how close to the root of the changes you stayed most of the time, especially in those places where one basic change lasts 2-4 bars. If you think of playing half notes in a "2" ballad feel, try creating slow walking lines to connect these tonalities together just as you would connect quarter notes in 4/4. With the way they sustain in a ballad, a lot of tension is created. For example, to connect the Dmi to the Gmi to the turnaround back to Dmi, the following lines could work (all half notes for now):
D...E...F...F#...G...F...E...Eb(or A)...D
D...E...F...F#...G...G#...A...C#(or Eb or other 8ve A)...D
F...E...D...C...Bb...A...G...(A or whatever other approach note you like)...D
Okay, pretty basic and you know all of that stuff. But then experiment (If the other players can handle it) with playing something besides half notes, and maybe making the rhythmic values not a steady symmetrical constant, but still resolving on the downbeat of your choice at the end of the line.
Of course on the bridge you can pedal Ab all the way to the end where it goes to C (and then C# if you hear that); it's a lot like the A section of Body and Soul on the famous Coltrane recording where the Ab works under the whole thing, or you can come out of it at any time, or you can string a bunch of pedal tones together leading to a resolution, etc.
Just a few off the top harmonic ideas, basically just filling in a bit on what David already said. Keep up the great playing!