It should do the planning in interactive manner. For example, first by asking what do you certainly want to keep (packages and directories) and then what you surely want to delete. The output will be a set of adjustable cleanup plans (the user should be able to compare the tradeoffs).
Idea for an opensource project: a disk space analyzer (like `dutree`) that is able to analyze not only directories, but also packages managed by various package managers. It should contain specialized code for each of supported PMs and give suggestions on which commands to use to free up space.
And also, some smart planning tool may be included, which might be able to answer questions like "I want to free up 100GB but keep firefox and my downloads folder intact, what to do?"