I wrote Margin[1] for similar reasons. But because I wanted to be able to incorporate notes as well as to-dos, absolutely anything can go between two square brackets in Margin:

[ ] denotes a to-do item

[x] denotes item marked as done

[anything] denotes an annotation of type "anything"

At first blush, it would appear Margin and [x]it! are compatible, with some very minor differences. Feel free to draw inspo from the Margin spec, and hit me up if you want to combine / collab.

[1] https://margin.love