Thanks for taking an interest in this. I'm sure there are a lot of other screenwriters who'd be interested if you get a good package developed. My main interest is just having a simple, uncluttered environment.
I've used LaTeX since 2004 until I started screenwriting and started using Fountain. The Highland editor is okay but I wanted something that would allow me to expand the language a bit. I tried Emacs and hated the keybindings, tried Spacemacs and loved the Vim keybindings but it was quite klugey so tried out Sublime but it's package for Fountain is incomplete. If someone is willing to work with me, it's pretty simple. The page for it is here:
https://fountain.io/
Priorities, in this order:
1) basic syntax highlighting (for me especially important is the three ways of commenting out text and making the comment colour dull enough that I can really ignore it). Maybe if possible a way to have the colours user configurable.
2) some very basic key transformations, e.g. shift+enter makes the line you're leaving all caps. Others could be done at "export" like Highland and Writer do.
3) ways of doing textual analysis. For example: "a way to extract the dialogue for a given character so we can see all instances of when, e.g. STAN speaks. This allows writers to see that the character is speaking in a consistent way and then we can compare with how JOHN speaks (perhaps in columns) so we can see that John uses different language etc than Stan. This would be a really cool feature normally only available on a program that everyone hates and is unreasonably expensive (Final Draft)."
4) exporting the text would be fine but optional because a screenwriter spends most of the time writing and export can be done in other environments; the Highland app has a switch to PDF view and the Emacs mode jumps the text into the formatted place (this bugs some people, maybe me; given the choice I think I'd rather it didn't jump the text around).
If you're interested, my computers are a 2016 MBP 13" with touchbar and a 2009 iMac. Other comments about this whole issue can be found by seeing the posts I've made under the same username on Reddit in the last month or so.