The inclusion of 'gathering and mingling' spaces in multi-family tends to be a function of the market--ie, when rents are falling, developers start competing on amenities. You can mandate the inclusion of these amenities by regulation. And, when the market is strong, developers can and will bear these costs. But bearing those costs requires spreading them over a larger number of units. Which means one of two things happen: The only structures that get built are very large multi-family units, or that the average number of bedrooms per unit falls.
Historically, the 'mingling' spaces of urban places have been semi-public spaces: Hallways, alleyways, small streets, and the plazas where two streets intersect at odd angles. Larger spaces (plazas, squares, parks) tend to be public, and publicly owned. Perhaps density added through infill of multi-family on single family lots should be charged a 'park fee' per unit, and that money used to provide more parks.
New York has managed to carve out a number of plazas by permitting developers to add the square footage 'lost' by doing so to the top of the building, although that has some limitations due to elevator capacity.