Good start.
This would be nice for a little loop, but as a song it needs some work.
Nice melody but rather robotic in structure. Some dynamics and increased range would liven it up. You mention adding a cello. That would be interesting because of the range of the cello and of the piano. Usually someone would add a violin or flute to such a sparse arrangement but cello would be nice considering what range you keep it in (or make it more virtuosic for the cello, which is also a viable option.
Out of curiosity, what are you using to produce piano sound? Is it a VST because it sounds very nice.