Oooh, very nice.
You're right, it's very simple, but I think that's part of the appeal. Whether intentional or not, I think the scales you pulled from, the slowness (not quite the right word... airy? graceful?) and simplicity of it, along with the lack of harmonic progression really pull together to create a unique feel to the piece.
I'd also just like to mention that intentionally leaving the harmonies unresolved at the end was a great idea and was a fantastic note to end the piece on (terrible pun intended).
That said, around the minute and a half ish mark (?) we'd kinda heard it already and it started to get a bit duller. To hit two birds with one stone (you mentioned you wanted a larger range in the melody?) You could step down into the bass clef with the melody around that point. You could use that "valley" to build straight into the climax of the piece, which I think would sound really cool.
One more thing, I don't think you should just entirely increase the tempo of the last minute and a half-ish. I think doubling up on the melody.. er... replacing each single note with 2 notes of the same pitch (with some exceptions, of course), and keeping the bass the same should be fine if you wanted to speed it up. (Personally I'd only do this around after 2:12 though)
But yeah, this is really nice. When you come up with other stuff I'd love for you to post it