Would be brilliant if he wrote a episode. Although he had the odd dud, his good one's were just amazing. Every one of his series finale's was brilliant, and there hasn't been one that has felt as special or as epic for me since he left.
Would particularly be great if he could work on the arc with Moffat. That way we could have Moffat's creativity, but also with RTD's creativity, but also his emotional punch and his ability to make it perfectly coherent which Moffat seems to have trouble with.