I completely agree, I would love to see a monopoly based tax system. I consider property ownership just a special case of a monopoly, since you have a monopoly on that plot of land. The one question you didn't broach here though is how you set the tax rate. Current systems do this on property by looking at the sale price, but property value can change, so they use an appraisal process. That's no good, it's expensive and prone to political manipulation. I'd prefer to see a market based solution. Imagine this: at any time you can make an offer on a piece of property and submit that offer to the government, the owner of the property can either accept or reject the offer. Offers are binding, so if they accept you're required to pay. But whether or not they accept this offer becomes the new valuation of the property and they must pay taxes accordingly. This should give you a system where the taxes paid on a property are always relative to the value derived from it by the party that values it most. And property should in general wind up being owned by that party.

The same basic structure should work for other monopolies like Pharma and internet infrastructure monopolies.