What are the legal requirements to open a crypto exchange in the USA if you don't handle US dollars? and possibly bitcoin?
First and foremost this is not legal advice, or by any means an exhaustive list of what you will need to do.
Second don't listen to me or anyone else about this and launch without talking to a specialist attorney that is very familiar with MSB regulation/filing on federal, state, and local levels. I recommend one that has worked within a government regulatory agency before as this can add political protection to you. Crypto Exchanges in the USA are not a simple thing anymore.
Under the United States Treasury Department Financial Crimes Enforcement Network (FINCEN) it doesn't matter if you don't deal with USD you are still facilitating currency conversions. Don't forget in the United States you are dealing with multiple layers of government and bureaucracy, just because one agency says one thing another agency can say something else and regulate. You will more than likely have to register with FINCEN, your state FINCEN equivalent, and possibly with your local government. You may have to file regularly scheduled reports till the end of time and maintain records on certain transaction types and amounts. This type of activity is called Money Services Businesses (MSB)and you need to obtain a BSA number form FINCEN.
Besides the regulatory aspects you need to be even more worried about protecting your clients coins!!!!!!!!!!!!!!!!!!!!!!!!! You will need to follow bank rules when it comes to your databases. You need have good security to protect against hackers.
How are you going to convenience anyone to use your exchange? Given the state of what has happened to other exchanges, its going to be hard for new exchanges with no funding to get off the ground. This is going to be a big challenge for any limited funded exchange.