The perpetual challenge of developing an iPhone-friendly gamepad (or any phone-oriented gamepad) is the bulk, either for a gargantuan case or else a separate controller. If Justice Frangipane's team and iDevices have their way, that clunkiness will be a distant memory. Their proposed FlipSide case for iPhones (we see a prototype here) centers on Bluetooth 4.0 gamepad controls that stay clipped to the back when just checking email, but attach to the front for playtime. They'll save us from hunting down a wall outlet, too; the combination of a sensitive solar cell and a thin film battery from Infinite Power Solutions should keep the case powered up through even indoor lighting. The only real challenge is getting the case produced, as Frangipane is looking for crowdfunding to make the FlipSide a reality. Provided his group makes its donation target, though, there's the prospect of an Android version -- so those who don't play the iOS way could still reap the rewards if they chip in at the source link.
Apple iPhone 6s