Of course, I've a little (maybe some would call it huge) project in mind with my 2 Kindle Fires. One part of that project is to potentially not use Android on the Kindle at all. There are various reasons for this, but the major one is boot time. In that project I can't keep the Kindle Fire in its well-working suspend mode, from which Android awakes really quickly. Instead it'll boot freshly each time and my plan is to get the boot time down to under 5 seconds.
So, currently KF#2 does no longer contain any Android files. It already boots in ~8 seconds to a command line prompt of a small Linux system, but my hope is to get that further down. The kernel takes ~3 seconds, but x-loader and u-boot are my main obstacles right now. u-boot is a mess to understand and Amazon didn't make the job easier by adding support for the Fire to it. I even considered to get rid of u-boot and boot directly into the kernel from x-loader, but that would also mean to miss some nice features (boot logo, fastboot, etc.). Yes, I still use fastboot and adb from android, mainly because I find them very comfortable to use.
I've built that small linux system using Buildroot and used the (non-Android) framebuffer OpenGL ES drivers from TI, though getting the SGX drivers working was tricky. On the other side the OpenGL ES demos from the same package and some own test programs run nicely already.