There's a great example of this in the Steam for iOS app. They haven't optimised it for iPhone 6 yet so what you get is a scaled up version of the default keyboard (in black!), with bigger keys and larger gaps, which works just great! I dunno why this isn't available as an accessibility setting.
If you want, you can use the scaled UI for the entire system by enabling it in Settings > Display & Brightness > Display Zoom. I personally find it too blurry.