As useful as the Google Experience Launcher (GEL) is, it's not exactly what you'd call "customizable." But intrepid modder Alexander Schulz has been working on that, making the GEL bend to his will through the magic of root and the Xposed Framework tool. The latest update to Xposed GEL Settings adds some really cool features. The most impressive is probably the dynamic home button: the home button on the virtual nav bar will switch to the app tray icon on the default home screen if that's the function you've selected. You can also change the Back button to a power button on the launcher. Neat!
The Xposed GEL Settings module now has full support for custom icon packs, a la Nova, Apex, and all the other custom launchers, but you'll have to upgrade to the premium version via a $5 in-app purchase to use it. Other changes include a handful of smaller customization options, like even-numbered dock icons, more scroll speed options, hiding icon labels in folders, and negative margins on the home screen. The dynamic Today Calendar icon is also supported.
Screenshots via this XDA-Developers thread.
XGELS isn't a stand-alone launcher: to get everything working, you'll need the Google Experience Launcher (part of the Google Search app for Android 4.1+) on a rooted phone or tablet, then install and activate the Xposed Framework, then install Xposed GEL Settings and set everything up to your liking. Xposed is free and available here.