Ideas and publication standards for Apps?

Now that I’ve written an app or two, for personal use and just for fun, I’ve begun to think about writing an app worth publishing. Are there any guidelines for publication, as far as code quality and standards and/or design guidelines to make apps broadly useful? Also, is there a way to submit tests scripting interactions and resulting states along with the app itself? (reading posts here it seems like apps do break from API changes - it’d be nice to have automated regression tests and an alert if a published app breaks…)

Also I’m looking for suggestions for what to write. My first app is rather silly, turning a hue bulb into a color-changing / blinking kitchen timer (or, time-out timer, really); and my second app seems like too many things mixed together - all my automation rules for a particular dimmer switch, so that I can temporarily suspend them if the switch is manually toggled; so I’d like to write something of more general practical use.