Comparing to another builder Khmer with their LT and EQ, Mississipians provide much less production and food, moreover it is invaluable  to place any district on river, as you lose food/production bonus from river tile. So my proposal for LT: the same numbers but provided from river tile (just like river infrastructure). So such buff will make them compitable with Khmer builder culture

Also good idea, at least it is more powerful than it is now

I agree their legacy trait is quite unimpressive. As you suggest, I believe making any district that Mississippians build on rivers keep exploiting regular Food & Industry resources that river tiles usually provide when they are adjacent to both Farmer's and Maker's Quarters in addition to bonuses coming from Flowing Water legacy trait, would be sufficient enough to make their trait at least relevant/viable.