You don't need undeveloped land. You could increase density on land that is already developed. In the US, even urban areas have a lot of spots where land is used inefficiently: surface parking lots, wide streets, large setbacks, single-family detached homes. Consider that residential streets in American cities may be 25m wide, whereas a residential street in a traditional city is more like 3m wide.

Detroit. Which is only a half joking answer.

In other places there are probably a lot of things that could be done at the planning level to make densification more likely. That's not the same as a cohesive project, but it might have a similar effect.