According to MacOtakara, a fairly accurate Japanese blog, the next iPhone could have waterproof and dust-proof features as Apple is apparently producing prototype 'iPhone 7' units that appear to be way more ruggedized than current models are.
In addition, the site claims that next year's iPhone will feature a new exterior design (we knew that, since 2016 won't be an S-upgrade year) and feature a newly designed non-metal chassis, which does sound interesting.