I have outlined how a complete replacement for centralized exchanges could look like:
https://bitcointalk.org/index.php?topic=4102739.msg38097740#msg38097740. It would be a mix of a Bisq-like protocol, stablecoins (like Dai or BitUSD) and atomic swaps (BarterDEX-like TierNolan-protocol-based or Lightning-based). The only thing it wouldn't support is high frequency trading.
Interesting proposal, BarterDex supports both Tether and TrueUSD. Integrating with an escrow service and then it's ready to go.
My proposal would use "decentralized" stablecoins, which have some incentive mechanism to maintain the price stable, but no centralized "backer" like Tether. The reason: these IOU-stablecoins are equally dangerous than MtGox. They introduce a single point of failure.
TrueUSD seems a centrally-backed stablecoin too, so it's also not what I'm aiming at. My proposal would be to integrate something like the DAI. It would be easier, however, with Basis/Basecoin, as this kind of coin has no "forced settlement" to fear.
@zaylynn: It seems that Faast is not a decentralized exchange but a centralized instant exchange like Shapeshift. At least, I wasn't able to find information about its inner workings (in fact, on their website they don't claim they are a DEX).