I have a mock up in Google maps here:
http://tinyurl.com/4xsfbdj

The only reason we need vehicle traffic downtown is so people can find places to park in the core. If people had places to park around the outside of the downtown (or if transit solutions were better - the dirty buses are 2.75 each way - cash only), the inside of downtown could easily go car free. Maybe try it for a month this summer and see what works and what doesn't.

I also see the an exposed riverfront that is open for bars, cafes, and bistros. Accessible only on foot.