In “Structural Basis for a Six Nucleotide Genetic Alphabet” published at the Journal of the American Chemical Society researches propose adding a couple of letters.
“Z” and “P” would contribute to the classic double helix structure with similar flexibility and rigitdy. Paper states that this would integrate well with current ‘conventional’ pairs. An ATCGZP DNA could evolve and take a life of its own. Literally. Including evolution and all its unforeseen consequences.
In theory, having an enriched set of building blocks would increase the possibility of creating new proteins and thus potentially useful for medial and scientific purposes. Perhaps one advantage is that such proteins (and other biochemical structures) is the fact that they would be easily marked and spotted as distinguished elements.