The easiest path off Heroku to self-hosted is probably Cloud Foundry: one of the inspirations was Heroku and several of the Cloud Foundry buildpacks are soft forks of Heroku's buildpacks.

Generally we find that any buildpack written for Heroku will work with Cloud Foundry without modification. With a little extra engineering you can create a buildpack that will run in a fully disconnected environment.

Disclosure: I work on the Cloud Foundry Buildpacks team on behalf of Pivotal.