Well the "extra rooms" I mentioned would function just like FC houses do with having that door that lets you select where to go. They would serve the purpose of an "outdoor" area for gardening and a "workshop" like FC houses have access to. But as I said if that's too much of an ask, SE could simply make new interior housing objects that allow access to those features (crossbreeding and airships/sub missions).
The primary issue I think many people have with apartments aside from size/item count, is that they are still locked out of certain features which are only available if you have an actual house. Simply addressing that, and perhaps offering larger apartment sizes and a higher furnishing count would probably stem much of the calls for more housing I think.