Essentially what are the legal requirements for starting the next Binance, Bittrex etc. When I say in the United States I mean US based customers (preferably registered in the US as well.)
I would suggest that if you are a US citizen that wants to get into crypto from an institutional perspective, you set up an offshore decentralized exchange that doesn't allow US citizens, this has a lower cost of entry. Unfortunately, due to various antiquated US government regulations, you are limited to the number of services your exchange can provide (such as peer-to-peer lending); thus affecting the overall amount of transaction and service fees one can collect. Once you are profitable with that exchange, you can think about opening up to US customers with a separate website, and base trading ability only. Overall, you are going to spend a minimum of 4x the startup costs in the US compared to more lax countries such as Cypress. Here are some links to get you started:
Best of luck. If you have any other questions or need any further help, please feel free to contact me. I am always available.