Here's a list of things that I think would not be controversial among economists and relevant experts but nonetheless seem very unlikely to happen any time soon:

• Much more free trade -- reduce friction, trade barriers, and tariffs. Consider payments to smooth out pain to short term losers
• Greatly reduced zoning and housing regulations. Housing stock is artificially expensive (at least in most places in the US, especially the Bay Area) due to excessive local regulations and zoning.
  
  Also no more rent control. Do you want to make sure there's not enough places for people poor people to live? Because rent control is how you make sure there's not enough places for poor people to live
• Carbon tax. It's the most efficient way to internalize the global warming costs. It'll never be adopted because the price sensitivity to gasoline is way higher than it is to e.g. electricity, so one blanket number will piss off consumers too much
• Greatly reduce drug approval costs. Accepting approvals from similar agencies overseas is one approach. Greatly expanded "right to try" rules might be another. A more free market approach might be best.
  
  Also make almost all (actually all?) illegal drugs legal. The enforcement costs and social costs are ridiculously high, far higher than the benefits from the war on drugs
• Payments for organ transplants. Supply is much lower than demand, and there's no price signal or reason for people to create more supply, so lots of people will die from a lack of a kidney while everyone else has a spare
• Charter schools/voucher schools. This might be more controversial among "relevant experts" depending on what group you think that is, but the arguments against are very poor and the arguments for seem much stronger to me
• Get rid of ~all tax deductions. The mortgage interest deduction is regressive and distortionary. The employer-provided health care deduction is distortionary and locks people into jobs. I personally benefit a ton from the charitable deduction but it's also regressive
• Get rid of the corporate income tax. We want corporations to make money and invest it. Tax income to people, not to companies
• Greatly reduce occupational licensing. Some of those may make some sense, but most are just thinly veiled job protection for the existing guild members
• Shorten copyrights (35 years or life of the creator, whichever is longer?), shorten patents, no software or business method patents

I don't think there's a single explanation for why none of those policies seems likely to happen, though at least there's substantial movement on the drug legalization front recently.

There should be no attendence requirement for any occupational licensing. Reduce all occupational licensing to tests that can be taken in a few days provided a person has the requisite knowledge. 

This should be enshrined by a federal right to work law. 

Drug approval denationalization would create an incentive to for regulatory agencies to be faster at approving drugs while still having standards for safety and usefulness for drugs. 

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Forced choice for whether one wants to be an organ donor or not. Part of applying for passports or driving licenses should include a form where a person has to chose whether or not to be an organ donor. This maximises both moral concerns of not taking away people's organs without consent and increases the organ donations over the default of opt-in.

Adopting Japanese style planning & zoning alongside European models of social housing organization & finance would unlock a considerable degree of economic growth in a lot of Anglophone countries. It'd also have the additional benefits of reducing the carbon intensity of housing and transport through greater density, and make efficient public transit easier to finance and develop. It also would in the long-term stabilize urban housing costs and reduce the precarity of low-income households, while enabling a larger number of people to benefit from higher big-city wages through more dynamic housing stock growth.

As it is, we are locking people out of the places where their labor is most valuable and where they would have the smallest environmental impact to the benefit of (relatively wealthy) incumbent property owners. This equilibrium is difficult to change when there's expansive local control over land use and housing development, as it's much easier for narrow coalitions of property owners to dominate those nominally democratic decision-making processes, and because local constituents have little incentive to take the utility of newcomers or the metro region into account.

There are indications that there are enormous deadweight losses in many sectors, which are not fixed because of political constraints. Eli Dourado wrote a great post about this: https://elidourado.com/blog/move-the-needle-on-progress/, and I wrote a short one for Works in Progress: https://worksinprogress.co/progress-studies-the-hard-question/.

There are various different social engineering techniques from public policy, political economy and related fields for smart policy design (not campaigning) that could be tried by policy entrepreneurs to engineer changes from these inadequate equilibria, but those techniques seem little known outside those fields. I'm working with a few others to summarize them and can supply more links if people are interested.

Make nuclear our main source of power. It's green, safe, sustainable, cheap and reliable. We could have done this in the 60's/70's as France did but irrational fears of nuclear power and subsequent over-regulation and lack of gov support killed it in the US and UK.

Instituting rule of law in foreign policy. In many countries foreign policy is essentially at the discretion of the executive. Insofar as it is controlled by the legislature, it's controlled through committees and reporting requirements rather than actually courts and rules of conduct. Imagine if the prime minister could choose to kill whoever they wanted and was only contrainted by the threat of parliamentary sanction. That's basically the status qou for foreign policy at the moment.