My own editor was based on Femto, initially. It's now entirely unrecognisable, and I've used it as my main editor for the last couple of years. I'm linking Femto rather than mine because mine is huge compared to Femto, and full of assumptions that only holds for me -- that was part of the purpose for me; I realised my .emacs file had grown to the point where I could literally write an editor in fewer lines.
That's not to say I'm suggesting that most people should write their own editor with the purpose of making it their main editor, because that will likely only work without turning into a massive time suck if you (like me) like your editors spartan and care more about ergonomics in a very simple package than "fancy IDE's". But writing an editor is fun because it can start very simple and you can gradually rediscover or reinvent all kinds of model-view abstractions, and get somewhere usable very quickly.