Let's say, you need to record a bass-line for the following group, and the song has SOOOOO much space.
Would you try to fill it with a lot of bass notes?
A beautiful song.
Not every song needs to be busy. Honestly though, this does feel a little lacking on the low end and I hear space for more bass (maybe soft, simple octave chords followed by a brief counter-melody after the vocals/guitar). I'd keep it very soft, especially the low chords; very organ like. Maybe bring them up more and add the fifth during the louder section. Or maybe try some dominant 7 chords there.
There are obvious moments of sound-texture where any line would be out of place. It is a very nice piece.