yes it is legal to create a new crypto currency in the market and then to add it in exchange, but after that to make it a successive one you need hard work for it, after that you will introduce it into people for this purpose you will use different strategy to introduce it to people so that they make take interest in it and do trading in it.
it's easy for us to make Crypto new, but it's very difficult for us to be able to introduce and be interested by many users, Crypto is like a product, so it's very difficult to market, more profitable for us to use something that is popular and proven safe and potential In the future, a good one, like Bitcoin, is already the standard of all Altcoins, why take risks is too big if what is certain is in front of our eyes.