This is a good question - I haven't studied much aside from densifying existing neighborhoods but when thinking about other options, I suppose suburbs come to mind. Before going any further, I should clarify that I live in the US and look at this through an American lens. Anyways, a primary issue with suburban affordability is that demand is high and space is limited[1]: many folks want to live in the suburbs but maintain their jobs in urban centers, and you can only have so much low density housing within commuting distance to a city. Since urban centers are where the majority of jobs are, it's tough to suggest that people "just move to the country", for example. Remote work could help with reducing density while allowing people to relocate to more remote and/or affordable places. More public transportation may also allow suburbanites to move further away (think high-speed trains and commuter rails).

I've also heard arguments for an urban model that focuses on smaller, more community-centric cities instead of huge urban centers like New York or LA. I don't have any primary sources for this, but I think the idea is to keep density low-ish, and increase the distribution of these urban nodes evenly across a region [https://en.wikipedia.org/wiki/Urban_village]. To me this sounds similar to the mundane suburban towns I grew up around.

Urbanity/density is likely the easiest[2] solution, but I'm sure it's not the only one. There are likely many thoughtful solutions that don't rely on density - I may look around to see if I can find any.

[1] I guess this issue applies across the entire spectrum of housing, which is why there's a housing crisis

[2] Easy is relative - obviously, this is has proven to be a very difficult problem to solve.