Cells

At some level it is similar to HMVC, and the general concept goes under many names. I think that HMVC extensions for CI tend to group together two or three concepts: partial views, View Models/Presenters/Adapters, and a modular directory structure (modules containing their own MVC directory structure).

One of the nice things I see in CakePHP's Cells is the isolation of scope, so the Cell doesn't see the variables loaded into the view (and the view doesn't see the variables in the Cell).

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide