i think the main problem is that you need the fastest response times possible to attract daytraders. You can't really manage that with a decentralized system at the moment.
Do you envision it for the future where it will be faster to get fiat in and out and to conduct trades?
I think without a centralized entity, it is impossible to have the fast response times you need, at least in the forseeable future. Any cryptocurrency needs validation by multiple instances, otherwise, you could screw the system hard.
A possible solution might be a way of IOU trading, where traders bind their coins in smart contracts that are managed and secured by multiple instances, while the trading itself is managed by a centralized entity, which provides an open, but centrally managed ledger.
Every few minutes, maybe 10 minutes, mabe 30, a snapshot is made, gets approved by the instances mentioned above and coins are being moved based on this snapshot. Ideally, this would function as a "killswitch": in case the central entity is attacked, the decentralized instances can shut down the transactions of coins, minimizing the amount of damage that can be inflicted.
Not to plug too hard, but this may be some usecase for this:
https://bitcointalk.org/index.php?topic=1689947.0