Since a centralized exchange operates a bit like BitSharesX (in that when trading you're not actually trading on the respective blockchain of the asset your trading) I think BitSharesX could be a full service exchange by including payout.
Since we already have node servers and chain servers what if we had asset servers. Basically it's like a chain/node server except it also holds the blockchains of other main assets like BTC and PTS. When you want to bring money into the exchange you ask an asset server for a deposit address in whatever chain you're coming from and it'll deposit bitAsset of the same type into your account, minus a transaction fee. When you want to cash out it's the same operation in reverse: You supply the external wallet address and this gets sent to the asset server which handles the transaction minus a transaction fee.
One big issue: The asset server's reserves on the other chains wouldn't always be sufficient to cover withdraws. Perhaps the client could have a "Convert Asset" tab and it'd list the reserves available of each coin supported by the asset servers. If a server doesn't have enough reserves for you then just do what you do now - got to an exchange.
We've been talking about this for a while. The question is how to do it?
For digital assets you can use Vennd.io to create a cryptocurrency vending machine.
For legal assets you would need a centralized Ripple Gateway.
We may also need a decentralized identity service but that is coming sooner or later.
This asset server method you mention, who runs the servers? I don't think it's a good method. It's too centralized and you're not explaining how it is decentralized or what makes it secure.
Counterparty is on Bitcoin so we know how it works. Mastercoin is on Bitcoin so we know how it works. Ripple is something we also know how it works. Why go with chain servers which no one knows how it works or whether it is secure? Can you prove the security model?