A federal housing guarantee. In every instance where someone cannot find affordable and sufficient housing for themselves and their family then they are entitled to the live in an unoccupied unit that meets their families needs (1 bedroom per kid and 1 for parents, at most 30 minutes by, public transit from their primary job, or car if they have a reliable vehicle and the income to maintain it) for at most 30% of their income. Their primary employer pays half the difference in cost and the property owner absorbs the rest. If this kills a business then so be it. Businesses are not entitled to impoverished workers.
Vacant properties should be taxed at an increasing rate sufficient to force the owners hand to either lower rates (and realize the losses from the new valuation from their creditors) or liquidate the property. The proceeds of these taxes can go towards the building and maintenance of affordable housing in the district their collected from.
This should encourage liquidation of speculative properties in hot markets and allow those without capitol to compete against those living parasitically off of their "investments".
Obviously this isn't perfect, and I'm open to admitting it's a terrible plan. But can we at least try something FFS? So long as people are homeless and can't afford to buy homes near their jobs I don't think anyone is entitled to profiting off of renting property.
Also worth trying:
Eminent domain land and task the national guard or army engineers with building minimally viable affordable housing condos. Attach a jobs program to train the un/under-employed from the area and scale up to the size of the problem.