You could tax assets, e.g. land-value tax.
Don't tax "capital" in the form of money, certainly don't tax labor.
Why punish productive effort? Punish use of resources. This will make people strive for efficiency and go a long ways towards eliminating problems like pollution, encourage recycling etc.
Real estate for example is very inflation sensitive and your tax bill would be all over the place if you taxed the capital gain on that. Then what do you do when the land falls in value? Do you really think the government would give a rebate? And then when it rises again, hopefully they wouldn't tax it again (the basis for the last taxation should have been raised), but I wouldn't put it past legislators grabbing for money by saying you can only back back 5 years or something to determine the tax basis.
What's your definition of capital?
"and like to cause a lot of social issues."
Which parts of this article on land-value tax do you disagree with?
https://en.m.wikipedia.org/wiki/Land_value_tax#Assessment.2F...
In particular, the "Value can be computed by capitalizing rental streams." (Is someone renting the land, or are they renting the office skyscraper that's on the land?)
"Further, valuators in the home-and-contents insurance industry do so in order to calculate an insurance premium, separating the value of the home from the value of the land."
My experience is that's done in a straight-line percentage method ("the land is 20% of the purchase price in this area"), which doesn't distinguish adequately enough for taxation valuation purposes, IMO, but does provide some basis for insurance company purposes.
"Another approach is the residual method: the value of the site is the property's total value minus the depreciated value of buildings and other structures."
So, the land I own under a rental property gets more and more valuable each year (as I depreciate the buildings and structures)? Then, I sell it to another investor and the land value drops again to a low baseline and starts to rise as THEY depreciate the buildings? That doesn't make any sense to me.
The article admits that the whole valuation topic is a hard problem and I agree with that.