The core of Nez is fairly complete. I don’t think it will ever be 100% complete, but at this point it’s pretty solid and very flexible. For example: out of the box, the renderer can be used in standard forward rendering, deferred rendering, multi-render target blends and pretty much anything else you would want to do. This was all added with no changes to Nez code.
As for Components to build on the framework itself, there are always more that can be added. Anything that is not game-specific for example would be a good candidate. Most Components are usually game-specific though so they wouldn’t work as building blocks in the engine proper.