This is something I've been thinking of developing myself as I'm looking for a new project to start and I don't like any of the journaling options available right now, foss or otherwise.
I'll look into Day One to see what kind of features I'd need to replicate/have alternatives for and see if it's something I want to dive into, especially because my journaling needs are very simple so my initial plan was to make something very simple.
Honestly I've never used Joplin. I'll look into it!