I was elated to see you are thinking of setting up a decentralized exchange. I've followed the progress of, say, B&C, but my perception is that they have become semi-centralized as they progress from vision to reality. Any interface with fiat will always be a choke-point, imho. I hope you solve it.
I think all 'decentralized exchanges' so far, and there are quite a few of them now, are semi-centralized - even where they use only crypto let alone with fiat.
A truly decentralized exchange between crytocurrencies would be too slow and probably too expensive in fees to be particularly useful for most people even if atomic cross chain transfer tech was built, so its a tough problem.
I also really hope a good solution can be worked out for this.
Everything's there for them to get off the ground, touring complete languages for interoperability between chains have been around for a few years but they're getting a big boost at the mo. Maybe it's one of those things that'll take a long time to get off the ground but when it does it will change market dynamics in a huge way, there already seems to be a big change in market characteristics since shapeshift took off and the more that grows and advances the more crypto will make traditional markets seem archaic.