They need trade goods and housing as in MoM
Exactly! There definitely should be a difference between "oops, I forgot to set something to build" versus "I'm intentionally idling my city because: a) I get a bonus of some sort for doing so, or
it's the end of the game and I just really want this to end".
I love the fact that my idle cities tell me now, but it can also be annoying if you want to queue up a bunch of production and then after making one choice the turn advances because it's no longer idle. It would be nice if it waited until I clicked off of the city screen before advancing.