to me it must be legal, as most of the people have created their own currencies, but to apply it in a state then it depend on the policy of the state, i think in most of the state it will not be legal to introduce a new currency and encourage people to use it for their shopping and in business also.
making new crypto does not violate the law, because crypto is different from fiat money that must be controlled by a country, crypto currency will only be used for certain commons, so it depends on the level of communication we have, because if it is to be used by a small community then we will make crypto new also in vain, we better follow or use crypto now, for example, bitcoin, which has been recognized worldwide.