Roadmap

Factorio is a continuous jam session.

Factorio is developed in a continuous development process. This means, that the targets change with the ongoing development.

From that article:

Any manager would probably say this is a bad thing, that we need a roadmap for the whole process from the start to the final release with all the contents, features and graphics planned ahead including cost estimates. I personally think, that the freedom of the jam (agile) way of development that allows us to react is the best for Factorio. We are inventing and extending the best ideas on the run, ideas that would never be visible in the start.

There is a roadmap thread in the forum, which is normally updated after each major release.

The forum also includes a big Ideas & Suggestions-Board which can be seen as the backlog for the development team.