I want to create a new cryptocurrency as an experiment. Is this legal?
My cryptocurrency won't use a blockchain, so laws specifically regulating blockchains might not apply.
The system will be completely decentralized. Mining doesn't create any form of control over another user's transactions, so a majority of mining being done by one entity wouldn't be a problem at all other than the possibility of them disrupting operation by just ceasing to mine, so a "51% attack" is impossible.
In general, the default is that it is legal to do anything unless it isn't. That's how cryptocurrencies happened in the first place.
Of course, you will have to follow laws that are applicable to cryptocurrencies and not do anything illegal with them or your business.
There are legal ways to create cryptocurrency, but it has to be done in a legal manner.
Creating a cryptocurrency is subject to various kinds of regulation.
In particular, an "Initial Coin Offering" is typically regulated in the United States as an offering of a security and regulated that way. Lack of proper regulatory compliance could constitute marketing an unregistered security. Consider, for example, this press release from the U.S. Securities and Exchange Commission:
Unregistered ICO Issuer Agrees to Disable Tokens and Pay Penalty for Distribution to Harmed Investors
Washington D.C., Sept. 15, 2020 —
The Securities and Exchange Commission today announced charges against Unikrn Inc., an operator of an online eSports gaming and gambling platform headquartered in Seattle, Washington, for conducting an unregistered initial coin offering (ICO) of digital asset securities. Unikrn agreed to settle the charges by paying a $6.1 million penalty, substantially all of the company's assets, to be distributed to investors through a Fair Fund.
There could conceivably be other regulatory requirements or violations of the law, depending on how the crypytocurrency was designed, was used, or its part in a larger plan, but this is highly fact specific and can't be answered in the general case.
