Drupal core has been lacking a fairly easy to use form wizard for a long time. CTools has a good example of how these can work, and with the advent of tempstore in core, officially blessing an approach is likely a good idea.
Config entities are the new way of creating most of our forms and storing new data. After working with them for a while, I began to notice similarities in the way they work and the way CTools wizard component works. With very minimal effort and a single class I managed to get a multi-step wizard working for my configuration entity.
The basic approach requires the developer to add "steps" to their entity annotation. The real magic lies in the WizardFormController which is just an extension of the EntityFormController which I hope means we can do this for config and content entities. I'm actively working with a config entity, but if we could start building up tests around this class, I think we could begin to illustrate how this works, and how easy it is.