While I am not in favor of an upkeep system for various reasons (for example: more grinding work for those who hate harvesting, does little to stop those who truly love to build, its anti-casual players and the concept will not stop spammers) there could be some uses of an upkeep systems if done properly.
This is all based on a single player (not making any suggestions for a clan at this time to avoid an extra layer of complexity to the discussion at this point)
Upkeeps should exclude placeables. Too many variables on placeables and programming to make it work.
For upkeep:
The first 2,000 building blocks should be free of upkeep. You managed to stay within this amount, great, you have no additional costs other than the usual upkeep from purges, raids and such.
The second 2,000 blocks cost 1% of the cost per 2 week cycle which is stored in new container (tax box)
Every additional 2,000 blocks will go up by 1%. In this case, they would be 2% for any blocks between 4,001 and 6,000. 3% for any blocks between 6,001 and 8,000.
So someone with 10,000 blocks for example will have to pay 200 blocks in cost per 2 week cycle. Granted the cost will take into account of the cheaper pieces, tiered materials, walls and such versus foundations, etc. but I just showing a simplistic view of the tax. Anyone who thinks 200 blocks is cheap tax is not looking at how much time and effort goes into making 200 blocks. Granted, it would ask for materials not blocks themselves, but it is still a process to collect and make. In essence, a progressive tax system. (8k would cost 120 blocks)
Why did I make it every 2 weeks instead of 1. As stated before, not to make this a too much of a grind for players / casuals. Exceeding 1,000 blocks is fairly easy for majority of players (as noted in another thread). Which is why I made it free for 2,000 instead of 1,000. We should not penalized those who rather pvp, pve than harvest/build.
This concept will not resolve spamming issues (no tax would). Nor it would resolve other griefing issues/methods either.
This concept or any upkeep system most likely will not be easy to program, and will possibly break 100’s of mods if not almost all of them, which is another reason not to make such a huge change. This idea is more reasonable than some upkeep ideas I have read. Well, food for thought. As stated in the beginning, I am not in favor of upkeeps but am willing to give a reasonable upkeep suggestion.