Here are some random thoughts:
I don't think the bass and piano supported each other well on the introduction. Hold off on the bass until the verse begins.
It's good to hear the vocals in front, but the instruments - especially the piano - are just too far back in the mix.
The doubled vocal sounds like two voices. I suspect that you want it to sound more like a single voice, but stronger.
I like the addition of the harmonies. More! And more parts!
Adding in the slide guitar makes for a good change in texture. If you could do something similar in other parts of the mix - changing things for contrast - it would help. For example, you could add strings on the second verse.
You're aiming for a big ending. How overdubbing a bunch "ooohs" and "aaahs" underneath, and 3 part harmony on the final chorus?
Hope this helps!