Actually the existing edit field is perfectly fine, but it looks like you’re all trying to solve a user preference matter on a site-wide basis. The solution is not to force your preferred editor on every user of the site you’re running (let alone every site running a Discourse instance), the solution is for users to be able to edit any text field with the editor of their choice.
Ah, I hear you say, but what if I want to use Chrome? Well, I don’t use it so I can’t verify this personally, but apparently this Chrome extension will do the job. It looks like it takes a little more to configure, but otherwise should be similar.
And what of Internet Explorer? Well, really you should probably stop that, but if you must … there’s this.
There are a few more listed here.
Oh, if Emacs ain’t your thing, that’s fine. You can run pretty much any text editor you like through these extensions. Editors that run inside a terminal may require a little shell scripting trickery, but otherwise you can do what you like.