We are using very advanced CloudFormation in the open source Convox platform.
https://github.com/convox/rack
I have touched every corner of CF including lots of Custom Resources.
Right now we are using the golang template tools and tests to generate our templates.
But I have lots of needs and ideas for improving this. A CF template compiler and simulator should be possible, giving us all tons of confidence in making template changes and therefore any infrastructure update.
I have some sketches that I haven't published yet.
And I strongly believe CF is the best tool in this space if you're all in on AWS. Let Amazon be responsible for operating a transactional infrastructure mutation service. It's ridiculously hard to do this right.
If you want to brainstorm some ideas send me a message :)