I think if they do get popular it will be like the slim-line Fancy Steel variety, so more like a bikini as opposed to a bulky pair of nanna underpants. I can see the plate on the sacrum becoming quite decorative, like in the Britney Spears visible panty line era. The opposite of a tramp stamp!
I'm not a big fan of the rubber / silicon tubing over the steel cables, but it's a necessary evil for now. Maybe there's a better looking way to protect the skin.
More choices in the colour of the metal, like rainbow coloured stainless steel, or gold, silver, platinum, black chrome etc.
High tech materials that are flexible but unbreakable, like some kind of military grade soft fabric where you can still move and stretch without being cut in half.
And like @Angelina said, smart locks will be the norm, with wifi connectivity, pin codes, fingerprint scanners etc.