50% vacancy tax. Any property that is vacant for more than 50% of the year would require the owner to pay 50% of the assessed value, unless they can prove there is zero demand for the property at any price.

Would solve the problem very quickly. It's a fair, equitable, market driven solution to the problem of real estate vacancies. But governments are much more concerned with maintaining the illusion of value, than effective land use.

There’s an old fast food restaurant near my neighborhood, boarded up, trash everyone, it’s a eyesore to the community.

Do you not have local ordinances requiring property owners to maintain them? Here's language in a local cities code:

All buildings, both existing and new, and all parts thereof, shall be maintained in a safe and sanitary condition. All devices or safeguards which are required by this Building Code in a building when erected, altered or repaired, shall be maintained in good working order. The owner or the owner's designated agent shall be responsible for the maintenance of the owner's building.

Not maintaining it would result in fines, which the property owner could pay, but the city could either keep increasing the fines to the point where its cheaper for the property owner to sell it.

Land value tax would fix this