You can get capital gains treatment on returns on investments in a given year which don't exceed 20% a year compounded annually. This would apply whether or not the taxpayer has realized the gain (though I'd be fine with an exception for startups to give them, say, 5 years to pay since liquidity events don't always align properly).
This solves the situations that enrage people, namely:
* Carried interest
* People that luck into huge gains on flyer investments (i.e. bitcoin or whatever
* Startup founders who get capital gains treatment because of the $100 (or whatever) that they put into the company initially
In each of these cases, these are really speculations, not investments. There's nothing wrong with that, but I don't see why it should get preferential tax treatment.