Should we integrate Flysystem

I voted no simply because it seems pretty simple to just add it yourself via composer if you want to. I feel like including this is just opening a can of worms where people are going to want other packages. Why include FlySystem and not include other popular packages like monolog, respect/validation, predis? I also don't understand the logic of including something like FlySystem because there's no point in 'duplicating' it by writing it yourself, yet Guzzle wasn't included and a very basic version of Guzzle was duplicated. Why not include Phinx as a migration tool?

One of the reasons I prefer Codeigniter to the likes of a Laravel is that it's small and very unopinionated.

I think you're playing with fire here by toying with the idea of using outside packages. Once one package gets officially included, there will be a large portion of people arguing on which other ones need to be included.

Codeigniter is simply one of the tools you need to learn to be a successful developer. Always add more tools to your coding arsenal!