You can do that right now in every modern socialist country, but what about the USSR and the Warsaw Pact? And any future socialist and communist experiment(s)?
By self-employment, I'm indeed referring to running your own business, but I'm referring to a sole proprietorship or freelancing, not your typical business where other workers are exploited for the benefit of the owner.
I'm not educated enough to answer that, but there is this interview where Aleida Guevara gives her opinion on it (it starts at 4:00).
She basically says that she doesn't like it because it promotes individualist thinking, and so should be avoided if the material conditions allow it.