good idea, i am thinking about it, but i guess it will be not that rich in formatting like the desktop app, the architecture design here using "vault" is a 'problem' for extend to mobile, maybe a lightweight re-designed version :-)
I would not mind less formatting features, in as much as editing on mobile does not mess with the desktop version.
Rather, usability on mobile would be a key priority for me. I find OmniNotes very nice for mobile. The primary reason to move away is limitations on synchronizing with desktop (I use SyncThing). On the other hand, I find Obsidian for mobile not very mobile-friendly.
PS: I had attempted to code such an app using Flutter (text-only first), however, ran into issues with security permissions on Android, and gave up after some 6-7 days of struggling with it.