Definitely Morrison's idea, but also definitely Johns' character development. All Morrison wanted was, and I quote the script to issue #1, "...the super-elite, the upgraded version - with hearts replaced by power batteries and implanted rings on each hand". Morrison probably only specifically used Kraken because she is from New Genesis, and allowed Johns to cherry-pick any others based on the character traits - Chaselon's misguided trust in the Guardians, Boodika's lust for glory, Green Man's search for a personal identity.
Morrison is credited with the idea in the issue preceding these scans.