Change functionality of Navigation bar in Android M I want to edit the functionality of my Navigation bar. I would like to change the "app switcher button" to "Switch to Last App" and long-press to be "app switcher button".

I know this is doable with Xposed Framework and modules, but I am running stock Marshmallow on my Nexus, so that's not an option. 

I believe I can edit System UI to do this, but not sure how or where to go from there.
Because you are running a STOCK rom, you can use xposed on your nexus. 
Xposed doesnt works on heavily modified roms, since nexus use AOSP and doesnt put intermediate layers, it is your best option.

Just go to xposed page on xda site
Download the framework (xposed-v80-sdk23-arm) and flash it
Then reboot and install XposedInstaller_3.0_alpha4.apk normally
Open it and in downloads section search for Xstana and XtendedNavbar
Install them and tick then
Open the modules in modules section 
Customize your navbar
Reboot
Vualá