Ink vs Harlowe 4.0
Leon, I took a look at Ink because you mentioned it in a conversation, and almost immediately I saw why you had reservations. I believe Harlowe is a ton of work, and it seems like you wanted to do something new like Ink. I can tell you Harlowe is my first love. I just appreciate something that I can understand the flow of, more than the other story formats. But, with that being said, I picked up Harlowe again after a few months off and the learning curve game back.
But what I really like about Ink is its simplicity - no mark ups (html, etc), just, almost pure words. Since it blends in with other programs like Unity, Twine shouldn't be a problem. It also supports using [brackets] as links which should play nicely with Twiners.
The way I see Twink or TwineInk (Twine with Ink): the first passage would be the full script (Script Passage) so users can just write their stories or games. In the diagram background, each Knot creates a passage and diverts link to other knots(passages) as they become available, and so on. Sure, we could have header passages for VARs, LIST, and CONST.
And since Ink already has it's own html & css code, these could appear in the appropriate panels, like stylesheet, etc for modification. And Ink's formatting is tight. For example, simply using <> joins two separate lines into one.
What I appreciate about Twine is its diagram scheme that allows you to see where the story flows or branches - because it's very painful to get lost and buried in a sea of text. Inky doesn't have that, but the more I use Ink, the more I want to use it. Sure, it doesn't have disappearing or reappearing text and fancy sliding texts as Harlowe, but it could. But in the end, the goal is just to tell a story.
But it appears you wanted to do your own version of Ink, perhaps Harlink or Hink maybe? But I feel, the more we keep to words, the better, cleaner and easier a story format will be to use. I don't know if you have started work on 4.0 as yet, but with limited time, would it be better to deliver 4.0 or create something simple, light, and easy to update and maintain?