Alternative Messages App, and separating Messages/Calls/Music/Maps

So I could make a new app that wrote to the messages.custom.js file, and it would use that?

Yes, that's the idea!

But... What you're proposing sounds like a great idea, and I'd be more than happy to replace the main messages app with that.

Just some thoughts on implementing it...

• You could do a scroller with each 'scroll item' high enough for 2 lines of text. That way you could still have space for the icon at the top
  
• It's been suggested that swipe left/right could be used to handle the relevant actions rather than buttons, and you could just mention this right at the bottom of the scroller (back seems like it's not needed now you can just tap in the top left or use the button)