It would be cool if you could also export/import different nichelings. Like, have a button on each nicheling to copy a code, and then be able to paste that code into the sandbox mode nicheling creator.
Also, to expand a bit on Cabinet's idea, maybe the random cosmetic variations could get numbers? The variations could still be random, but you would be able to put your desired variation into sandbox mode, maybe via a dropdown box or hoverbox. Little concept with the poison body gene, since the variations on it are random. As an example, maybe variation 1 is green, while variation 2 is purple, and so on.