I think BitShares is what you are looking for. They have something called BitUSD that you can trade back to and hold value in dollars, if you'd like. You can also use the Bitshares you own as collateral and borrow BitUSD. In that way, BitShares is like a bank... but that you hold the keys to it.
I started using BitShares a couple weeks ago and it's awesome!
Bitshares is indeed a decentralized exchange. And it IS very close to what you are looking for. However, the volume is much lower than traditional exchanges, and in times of great depression, there is some fear that converting OUT of BitUSD could theoretically become difficult if not impossible. However, we are not in one of those times. I would just sort of caution you not to keep all of your assets there. Diversify, always : ).
Diversification is good. It is easy to move Dash, LTC and BTC on and off BitShares using OpenLedger. That means that you have to keep crypto in those currencies, as you may loose a lot trading to to them in order to get them off.
However, you can also use services like ShapeShift.io to convert BitShares to many other crypto-currencies.
The liquidity and volume problem is one that will hopefully be solved by posts like this (thank you @heiditravels) that promote decentralized exchanges and get more people to use them.
Ohh any fees charged? But they only offer limited amount of coins right? How would you compare with centralized one?
Fees per trade are around a hundredth of a Bitshare (so way less than 1 cent). There are a limited number of coins, but still plenty of good trade opportunities.
I can't compare to centralized exchanges because I made the decision early on in my "crypto-journey" to primarily use decentralized blockchain technology. So I have never directly used a centralized service.