Do you see any overlap between something like Roam and this?
If I were building a v1 of this iOS journal, I'd do the following:
1. Consolidated daily and weekly ledgers in a nice UI, basically a much fancier version of one of those generated Photos albums. Lets us scroll through to see our day with objects representing the activities across all the different apps we used.
2. Ability to attach a journal/note on anything in those ledgers, both individual objects (e.g. photos, a health reading, a convo in messages) and the overall day/week.
3. Reuse Notes' UX and formatting for these journals. No need to link between notes for v1, that's a power user feature.
4. Ability to attach journal entry from within individual app experiences like Health, Notes, Calendars, Photos, etc (possibly via Share Sheet, but perhaps more embedded), as well as link back to the main ledger when there are journals attached to those objects.
Honestly hearing that Apple is doing this makes me want to join them and work on this product. In my eyes, journaling needs to be an OS-level experience, because greater affordances for self-reflection are a core part of developing better executive function. Making reflection just another app competing against all the other apps and addictions and bullshit that our devices provide is a disservice to our sense of agency and self-determination; journaling and reflection need to sit atop these experiences.