How about a way to either avoid taxes, or get some of it deducted for big taxpayers?
Tax avoidance, for example, would involve people with 200k worth of properties somehow trying to avoid taxes altogether (it being only 1600 bucks anyway) at a certain risk like facing double taxes for the attempted evasion, somehow. How something like that would work, I don't know, but the idea of tax evasion through script would be a nice alternative to people selling properties to avoid taxes, which I've heard developers call script abuse. On top of that I heard it's impossible to evade taxes like this since the last update. Either way, tax evasion somehow would be nice.
Tax deduction is another idea I had for people who own a total of 2 million or more worth of properties. It would involve somehow being able to get discounts on the taxes to be paid. It could be done through the economical court in some cases. We could come up with regulations and laws on the matter of how exactly the tax deduction is to be done. The reason I think this would be nice, is for the people who own the really expensive stuff in San Andreas, such as Mario Soprano's Avispa, which at a staggering 13 million value, is gonna cost him 102K a month, which is 1224K a year. I think this is a bit over the top, so ways to deduct taxes would be a good idea for those really big real-estate owners.
Another thing I just came up with to counter those insanely high taxes would be something like the Dutch income taxing system. It probably works differently in every country, but in the Netherlands, the amount of taxes you pay over your income, is basically based on the amount of said income. The graph below gives you a picture of what that looks like:

Now, I think it'd be nice if we change the amount of taxes to be paid for properties, based on the height of the cumulative property value a player has to pay taxes over. It would look something like this:
Total property value Amount of taxes paid over relevant prop. value in %
1.000.000,- and less 0.008
1.000.000,- to 2.000.000,- 0.007
2.000.000,- to 4.000.000,- 0.006
4.000.000,- to 8.000.000,- 0.0055
8.000.000,- and more 0.005
These numbers are something I think would be fair, but I'm just trying to illustrate what the system would look like. In income taxing IRL, it's actually the other way round. The more you earn, the more you pay. In Argonath, I think it should be the other way round because of the way the economy works. High-value real-estate owners don't typically earn more than the rest of us. Everyone pretty much has the same oppertunities in Argonath in terms of earning cash. This is why I think people paying 100k a month just to avoid foreclosure is a bit over the top. Sure, they should pay more than if they were to own a shack in East Los Santos or something, but it's gotta be leveled better in my opinion.
Of course this would also open a bunch of oppertunities for court and mayoral roleplay, actually allowing people to form an opinion over a certain tax plan a candidate may have. It also puts the candidate in a spot where he has to come up with a concrete plan on how to tax people, rather than just promising ''LOW TAXES''. It all depends on how serious you wanna get about it. Maybe I'm overcomplicating things, but it really could open a lot of doors if the community's already on that road. I mainly brought it up because of those spectacularly high monthly taxes for certain players, though.
Also, since the interior price is deducted from the total property value, it will also make those people on the edges of 1 mil, 2 mil, and so on and so forth, think about what kind of interior/furniture to put in their interiors to stay in the lower taxing regions. This is something that actually could introduce a whole new angle to owning properties in Argonath. Those who get the system can profit through deduction, avoidance or just being clever in managing their properties/interiors, while those who can't be arsed will pay the 'standard' tax. It's about making it fair but also keeping it interesting rather than just paying a set amount into the mayor's pocket.