Ripple system is dependent on XRP's but a completely decentralized exchange would not have a coin. Instead it would be a issuer who is created out of chain of trust entrusted by issuing token's. The tokens can be used to create and issue Financial instruments. Which can be commodities or currencies or even tickets based on dynamic pricing etc. The issuer can issue the financial instrument only ones he has created. The created FI's are granted to Traders/ Users. The users Broadcast the open orders for One Financial instrument to exchange other financial instrument. When a countering order is met then a execution order is generated which is verified by miners and complied into block chain. The execution order determines price of exchange for two financial instruments. Similarly the market exit could be facilitated by Issue in terms of withdrawal by traders.

Moreover ripple is kinda private controlled. A Dex market system is not meant to be privately controlled but open for all.

 
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