I initially found Immerse Me (xposed - http://repo.xposed.info/module/com.mohammadag.immerseme), but it forced all screens to be immersive.
Then I checked out the xposed module App Setttings (http://repo.xposed.info/module/de.ro...ds.appsettings), and found that I could force a full screen on a per-app basis (under "Full Screen").
All apps are initially set to default.
This xposed module also does quite a few other things - letting you set permissions and such on a per app basis.
This has helped quite a bit - gets rid of the softkeys for most apps - view with a simple swipe up from the bottom of the screen.
I know there are apps out there (paid) that will do this for non-root users, but if you have rooted and like xposed (like I do), this is a great feature.
Why HTC went with softkeys for the M8 is beyond me. The screen is essentially the same size with the softkeys as my One X with capacitive keys and smaller screen.
This immersive mode tweak actually lets me use the entire 5 inch screen, rather than sacrifice .3 inches for the buttons.