* custom allocators and EnTT allocator-aware in general (long term feature, I don't actually need it at the moment) - see #22
* scene management (I prefer the concept of spaces, that is a kind of scene anyway)
* review doc: separate it in multiple md/dox files, reduce the readme to a minimum and provide users with links to the online documentation on gh-pages
* debugging tools (#60): the issue online already contains interesting tips on this, look at it
* dynamic view, useful for runtime ecs, can be filled with the desired pool at runtime and are not constrained to a compile-time list of components
* define basic reactive systems (track entities to which component is attached, track entities from which component is removed, and so on)
* define systems as composable mixins (initializazion, reactive, update, whatever) with flexible auto-detected arguments (registry, views, etc)
* create dedicated flat map based on types implementation (sort of "type map") for types to use within the registry and so on...
* does it worth it to add an optional functor to the member functions of snapshot so as to filter out instances and entities?
* ease the assignment of tags as string (use a template class with a non-type template parameter behind the scene)
* is it possible to reduce the storage used to manage empty components?
* reintroduce meaningful copy/clone functionalities into the registry
* is registry/utility.hpp really required?
* "singleton mode" for tags (see #66)
* AOB