I mean, you can technically use the mesh pieces for a roof - but they are fairly limiting. Adding mesh roofs would be an amazing, simple way to give us more flexibility. Netting is a must-have for aviaries too, IMHO.
I'm not sure that we'd need incubators since birds probably wouldn't lay eggs anyway (devs have previously stated this). But for me, we'd want a better variety of foliage as well as more in the way of logs/sticks that could potentially be used as perches. Smaller water effects features. Placeable pool/pond pieces would be really nice as well, utilizing concrete or plaster.
These are all "nice to haves," and obviously when you actually add the birds you'd need more (new enrichment items, nesting boxes for shelter, etc). But the game has plenty, as it is!
Not so sure we'll get animals laying eggs. The devs have previously said they weren't going to do this simply because of the resources it would require... For such a small/short part of an animal's life.
FWIW, though, the eggs that were included in the game seem to map on to animals that are already in-game when you look at them size-wise. I still think it was an odd thing to add--that feels kinda "hint-y" but they do seem to match onto animals already in-game.