Your zero fee offer is spot on. It is a real pain when rapid trading and impossible to get a sum. I really would like to see how you are going to manage to provide instant withdrawals, sounds exciting. But the real winner is, believe it or not, 24/7 telephone support. If you really pull that off, you'll be the first exchange. Just as a side note, that can also be chat support. It is usually more practical than phone support, I think.
As you know there is an ongoing talk about a possible economic crisis with ramifications including a loss of value in fiat currencies, especially for EURO because of vulnerabilities regarding eurozone. Some thinks that this might lead investors to cryptocurrency. Considering the whole thing can occur anytime, the sooner you open the exchange the better.