Really interesting stuff. It sounds very similar to what NXT was doing (and probably still is) on their platform. I did a ton of trading on that platform a few years ago.
I don't know if you are familiar with NXT, but they have the ability to generate side chains on demand and people create tokens to trade on their open market in what sounds very much like this.
The biggest two differences I see that Steem can leverage is the community and timing. I think NXT was too early and never got the exposure that Steem can get. Obviously Steem has a social aspect and that is the catalyst that all other platforms are missing. Steem really feels to me like it is gaining a lot of attention lately.
A Steem based DEX will be fascinating for exactly the reasons you said. It will be really interesting to see how much and what kind of use people will come up with, especially now that things are beginning to mature to a point that at least more and more people will be willing to try it.