I'd like a project setting to replace points with average # days spent on stories, and use that as the prediction mechanism

This would hide the estimation capability / points stuff, and instead count days elapsed since clicking start. I need a place in project settings to say how many parallel story slots I have - let's say for example "1.5". A rolling 3- or even 6-week average of days-to-complete-a-story is computed, and that tells me how many stories I'm likely to get done in the next iteration and out into the future...if my average-days-to-complete is 3, and I have 2.5 story slots, that's 2.5*5 slot-days = 12.5 / 3 = 4 stories predicted to be finished this iteration.

So you have yesterday's weather prediction, and people clicking start and finish and accepting, etc - using a lot of the core parts of tracker just without up-front estimation and all the stuff about points.

I can't imagine this would be hard to accomplish and is well within the spirit of Tracker.