I think it sounds pretty good the way it is. If you are going to add additional instruments, you will definitely need to carve out space in the mix, but this sounds pretty good as it is so far. The piano sounds a little dark, may be your intent, but I'd like to hear more air on it...
One suggestion, if you want to experiment, would be to move the piano more towards the center and place the strings around the piano (left and right). Then send the piano to a bus and add a tempo-based delay with modulation and fade this in and out to add some rhythm...not too much... You could even make that delay move around the space by sending that bus to another bus with another delay with a different rhythm. Lots of things you could do here...
Regards,
Dan