I was just musing over possible directions to take metaexchange in the future, one of them was to move most of metaexchange functionality onto the DEX (order matching, market making etc), such that the user would then have the convenience and speed of using a centralised exchange (confirmation times are a PITA for users, and the METAEX.IOUS are basically instant), while at the same time having at least some of the security of a decentralised service.
The only real hitch to this idea is the transaction fees for transferring funds, placing and cancelling orders. It would be totally impossible to explain to users of metaexchange why they need to pay a transaction fee (of any kind) to place, or cancel an order. The fact that canceling refunds the cost of placing the order is not enough to make this frictionless.
I actually found myself discounting bitshares as a way to achieve decentralisation of metaexchange, and pondering forking the entire blockchain to remove the fees, but that is ridiculous.
IMO, it would be great if we could just remove all fixed size fees and replace them with a PoW submitted by the sender, which will eliminate spam while making transactions zero fee (except order fills, that is an acceptable fee for any exchange).