I'm responding to the prompt in the OP, not the title.
> What's a piece of software you find essential that you wish you could replace or rewrite?
Emacs is the most essential piece of software in my workflow. It's probably not the worst, but it's the one where I see the most room for improvement. I lean heavily on org-mode for tracking what I'm working on, it's like my command center. I keep two emacs frames open at all times, one dedicated to org-mode.
Supported by an assortment of magical packages like helm, projectile and magit, I write code and anything else more efficiently than any other editor I've used. I was a vim user for ~5 years, and now use evil mode for modal editing in emacs.
So yeah, my opinion is that emacs is the best editor out there. But honestly it takes too much time to configure and maintain. I spend that time, because I don't feel like taking the productivity hit that I would by switching to another editor, but I wish I didn't have to.
I have a vision for a SaaS app that hosts my emacs config and provides me a nice graphical, discoverable interface for managing my configuration. It would have simple, intuitive flows for setting up the essential packages. Like maybe I could scroll through a list of the most popular packages (helm, projectile, hydra, magit, etc), and click to install.
The current state of the art in managing emacs config is googling the name of the package you are trying to configure, and trying to find someone's blog or github from which to copy/paste code from. There has to be a better way.