Depending on what language you are using google's app engine could be a quick replacement.
http://rogerstringer.com/2015/05/13/make-your-own-heroku/
https://github.com/buttfoundry-community/bosh-buttfoundry/bl...
Another HN thread: https://news.ycombinator.com/item?id=4183634
I'm not a big fan of docker for a few irrelevant reasons but using it w/ heroku makes it very easy to migrate to other container hosts.
edit: the founder of Flynn replied to parent as well, that is also a great alternative that I forgot to include.
Heroku is a great product and it going down permanently would be a huge loss to the community.
However there are a number of open source alternatives with very similar user experiences that you can run yourself in a variety of environments including:
https://flynn.io/ https://github.com/dokku/dokku http://deis.io/ https://www.openshift.com/