For the intangible things I would say that an open bidding process every census period would be a good way of judging what the market thinks it's worth. Adjustments might be necessary for changes in the constitution of an asset. (E.G. There's now a building, discovered natural resource, or it's part of a different sized lot unit.) Approximations in resource description could be used to round up/down and group together the units in an area for some anonymity and consistency.
To prevent collusion in 'sitting' on an area those who own it would also be required to bid in buying it back. If they come out over the median bid then they get their land back (but are taxed at the rate they sold it for), if they don't then they can keep the land but get taxed at the 95th percentile bid rate for that area. The top 1% of bidders would also have the option of buying any asset forfeitures within that area at the price that they listed. That would also be the assessed tax value of that land for that period.
If you did this you would not need a very high tax rate (my guess is something around 1%) and it would encourage efficient allocation of assets. Of course the owners of all these assets won’t be happy and since they are very powerful this idea has zero chance of ever being adopted.