The most radical change that is needed is a land value tax and when consider that monopolies should pay tax for the privilege of withholding something from society it is only fair. It isn't called the "unearned increment" for no reason.
An equally radical example would be a Wealth Tax, which would have a similar justification if one considers the wealthy to have a disproportionate amount of influence on the government (and the government has a monopoly on sovereignty in its territory). Another way to look at it is that those who benefit the most from a society (measured by their wealth) should pay back to that society in proportion to that benefit.
In fact, many successful countries already have a Wealth Tax, so it isn't such a radical idea[0], but I would argue that the tax rate for such a tax should be equal to the equivalent tax rate that the median citizen pays, as a percentage of their wealth. For example, in the US, the median household pays about $10k in taxes per year, and has a net worth of about $120k. That means that an equivalent Wealth Tax rate would be about 8% per year (with maybe a tax free allowance on the first $10 million).
https://en.wikipedia.org/wiki/Unearned_increment