As things currently are, housing is basically a limited resource that can only be gained from certain buildings, and can only become plentiful once neighborhoods come into play. My idea is that there should be a building project that adds more housing in the city center that not only costs a sizable amount of production, but also reduces amenities in the city. It only makes sense that having crowded housing in the middle of your city would make people more unhappy. From a gameplay perspective, this would allow you to turn your excess amenities into housing, which would buff amenities (Which currently feel so easy to get plenty of) and effectively allow you to get as much housing as you want at a cost. This would of course be meant to be a source of housing that you would only want to go after when you've used up your other potential housing sources, and would basically be a last resort. A great thing about this would be that you could finally make high population cities in places where you don't have room for neighborhoods, and would mean you can make megacities for the fun of it without needing neighborhoods on every tile. It might also be a good idea to buff neighborhoods along with this, since neighborhoods are generally not used very often anyway, so it would be nice if there was an incentive to have maybe a couple neighborhoods in each of your cities.