Lets say you offer a bounty, find a person or team, and they build the software and collect the prize. Then what? They just leave? The software would need constant work and upkeep, the exchange would need vision and leadership as the market and Bitcoin itself change with time, there would be ongoing legal and press tasks, and so on and so on.
You... Just aren't getting our needs, bro.
What you're talking about is the very definition of the word "Centralized."
The number one criteria for this project... A project that is intended to protect the price and even the existence of bitcoin through any kind of government assault... Is DECENTRALIZATION.
It HAS TO operate like bitcoin, with it's own dev community around the world and no ownership or keys for uncle sam to swoop in and swipe.
Think of it as a bitcoin-like, peer-to-peer client program that is both a secure wallet and a book of orders. The only challenge I really see is getting Fiat money into that wallet... That's it though. Bitcoin and other cryptocurrencies can obviously get in there without any fuss, but getting USD or other nationalized currency in it is the real stumbling block.
Because the order side is actually pretty obvious. Each client has a real-time book of orders, and when the client turns on, it searches for all of the other clients within an acceptable Pingtime of it, and announces itself to them.
At that point, you see any orders on your book as well as all orders on the books you're within range of.
Let's say you're in Baltimore. John down in DC and Mary up in Newark have placed trade requests to all books within range, and want to make a trade with each other. Since you're inbetween them and happen to be the node with the shortest overall traceroute, your client is agreed upon by all parties to be the one that "makes the trade" and therefore gets the fee.
...The fee part is especially powerful, because similar to mining, this means that everyone who just keeps their client software turned on can earn passive income! This would be such a powerful incentive that it alone could make the whole project competitive to the best centralized exchange like Mt.Gox.
Anyway, that's my elevator pitch for what bitcoin needs to survive. Sure, it's limiting trades to regions instead of the global trades that Mt.Gox and others enjoy today; but not too many people believe that centralized exchanges will even be allowed to exist tomorrow.
Besides; who wouldn't want to use an exchange totally anonymously, like bitcoin itself? Isn't that the bitcoin promise, finally delivered?
So in my opinion, it's just a matter of time before a completely decentralized exchange is the normal way to trade cryptocurrencies. Governments will force us to make it, and this damn price volatility we have from all the DDos attacks & the central point of failure (MtGox) pretty much necessitates it as well.