Or the city could just invoke eminent domain, buy it at a very low price if the owner can’t demonstrate it having a high market price, restore the building and democratically choose an equitable process to rent it or sell it, hopefully favoring existing citizens.

But of course, we’d have to be a lot more aggressive about invoking eminent domain for this to be effective. To that end, it would make sense to also establish policies ensuring it’s easy to invoke it fairly and transparently - ie, easily allowing other parties to bid against the city for higher prices in cases where eminent domain is to be used this way (stipulating that those parties also take measures to restore the building, etc).