I honestly believe that the most important thing about the exchange is security including customer privacy. You can also add some promotion to your exchange maybe to increase volume. Another thing that is really important is tiny fees. Many exchange fees on this day are so ridiculous. Good luck with the exchanges and I hope you'll provide us with anything better than we have now.
There are a lot of exchange in the cryptocurrency world now. It is hard to choose which of them is the best, I think if the OP will make his own exchange he should have a unique idea that the top exchanges that are listed in the coinmarketcap dont have. Having the best security is not enough now, even binance and others has it, a better idea and more unique one is needed, together with a good dev.