1 - Since everything is new, this doesn’t really help. As the feature matures and more media is added in the future, you will start seeing “New On”, “Newly Added”, etc. type of hubs.
2 - Nice idea. I’ll pass it along.
3 - I think that is already being considered, just not implemented yet.
Well looking at my play next channel it only has a single item from Plex, while my On Deck in Plex has three items.
EDIT: Oh wait. After launching the Plex app again “Play Next” has updated to show the new On Deck items that were added yesterday. Not sure why the delay or if all home screen channels are dependent on the app being launched in order to update.