The coin would use smart contracts to tax all transactions but the tax rate is less for those who hodl less money (have less in savings) and for those who spend more per month (basically giving a leg up to those who spend like the poor, i.e. spend most of their money monthly...
if you combined this w/ govt georgist LVTs I think a fair system could be devised that still has people who are wealthy, but maybe 50 million is the cap, and maybe it goes up to 60 million when the average amount of monthly carryover in accounts is 10k, or something...encouraging more money to flow down, so the caps can go up... Something like this would need to figure out something else for business accounts to get ecommerce flowing and moving in/out of the system, since businesses can easily spend a billion a day or bring more than that in...
Haven't thought that far ahead. I'm also not a blockchain dev...so outside my wheelhouse..
[0] My main reason: there are countries such as Persian Gulf oil sheikhdoms where the citizens get free money or a least well paid jobs-just-in-name that someone else is actually doing, but there wasn't any explosion of personal freedom, social progress or artistic creativity there; a lot of the free money is sunk into drugs and conspicuous consumption.
[1] My next reason: you would have to exclude freshly arrived people, because if any place in the world becomes known for giving significant money to anyone for free, it will become a target of worldwide UBI tourism on an unsustainable scale. Already the people smugglers in Libya etc. are very good at presenting themselves as the "bridges" towards life in the richest parts of Europe.