When I learned that Simon Stagg was going to appear in The Flash episode "The Fastest Man Alive", I thought for sure that the superhero Metamorpho would appear and Stagg would be responsible for his origins. If Eobard Thawne, under the guise of Harrison Wells, hadn't stabbed Stagg to death, what would that have been like? Would we have gotten to see Metamorpho? I feel like that Metamorpho should've appeared because he was referenced in the Arrow episode "Broken Dolls" as a company condemmed after the Earthquake in the glades caused by Malcolm Merlyn. I don't mind that Kyle Nimbus got to appear, however, I have a feeling that Metamorpho should've made an appearance.
I have a theory that if Stagg had lived, he would've still tried to go after Barry so he can study him feeling that something inside the Flash would be able to make him more money. Eventually, Simon discovers that his daughter Sapphire is having an affair with a Stagg Enterprises security officer named Rex Mason, so just like in Beware The Batman, upon learning that Mason was having an affair with his daughter, Simon posed as an intruder or gets someone to do it in order to lure Mason into a gas tank and expose him to gasses from an experiment, codenamed "Project Metamorpho". Even though Mason survived, his appearance changed, giving Simon the pretext he needed to disapprove of Mason's relationship with Sapphire. Simon deletes the security footage of his sabotage, however, it gets discovered by Eobard Thawne while impersonating Wells. "Wells" tells Mason about the accident in order to protect Barry and get Mason to kill Simon while working with Caitlin to form a cure. Barry, having figured out that Mason is Metamorpho, attempts to have him cured only for it to not work. Simon tries to pin the blame on the Flash, however, Flash shows the footage of what Simon did to a devastated Sapphire making Joe having to arrest Simon. I felt like Barry coiuld also defeat Metamorpho the same way he defeated Kyle Nimbus, AKA Mist, in the episode "Things You Can't Outrun". Sapphire, while being devestated, runs her father's company and eventually sells it to Kord Industries while still having a stake in the company. I wouldn't want to completely copy Beware The Batman in making Sapphire Stagg a villain. I also have that feeling that Sapphire knows Barry & Iris due to having gone to high school with them. Sapphire and Iris could be best friends or girls that just dislike each other. I can honestly see Iris being popular in high school while still being nice to Barry and Sapphire being Iris' best friend. Anyway, Mason would eventually return and start working with Team Flash as an ally or work with Team Arrow while hiding in the sewers of Star City. Mason would get the nickname Metamorpho from Cisco (I can honestly see Cisco naming him that).
Metamorpho could also appear in Legends of Tomorrow' and be an explorer just like his comic book counterpart. There are always Staggs involved in his transformation, so how about something different? Rex can be introduced an an adventurer hired by the company to retrieve the Orb of Ra. In an Egyptian pyramid, Rex Mason is knocked out by tomb robbers and is evetually exposed to a radioactive meteorite from which the Orb of Ra was fashioned. The radiation then transforms Mason into Metamorpho. He gains the ability to shapeshift and change himself into any element or combination of elements found in the human body. Later, Rip Hunter discovers that Mason is but one of many metamorphae, created by the sun god Ra – using the meteorite – to serve as warriors in his battle against the god Apep, "the serpent who never dies."