Hey there. Love this tune. As you say, it's pretty full and it's hard to add much but .....
I added the sound of waves breaking gently on the beach (since that's what the mood of this tune made me think of). Added some congas for a little light relief, then some additional acoustic drums for the second half to pick up the momentum a bit. Not sure about the FX that I added, but the siren near the beginning of the original sounded like it might be lonely on its own :)