In your world, when one doesn't sell land, but merely the right to buy it, it should be taxed as if it were a land transfer?  That defies logic.  Any profit made by selling a right-to-purchase is taxable under present income tax rules.  But if you don't sell land, how can you charge a land transfer tax? 

Using your logic, if you buy pork belly futures on the stock market, you'd have to pay GST on the value of the hogs you have the right to buy, even though you haven't bought any hogs.