To give some additional context 1) we will be flying overnight from the US in business class so will “hit the ground running” our first day in BA; 2) our pace during trips is much more rushed than most - we are much more “see everything we can” than “slow down and absorb” people; and 3) one of the things we really enjoy on trips is going to local markets to see people as they are taking care of their day-to-day needs.
It is in this context that I was not discounting the feedback of those saying that we are trying to do too much - for us, 3-4 destinations in 8 days is just fine. In thinking of the two side trips - “hot, dirty, and messy” is for us appealing if it gives a window into how Brazilians and Argentinians stock their homes (being mugged is of course less appealing).
The above would give us some more time at the falls as well as an extra day around Rio for a couple of the great suggestions from VidaNaPraia. I was thinking of the two part-days to Uruguay and Paraguay as a quick fix of something different from the three main stops (trying to work in a stop in Chile or Bolivia involves too much travel time), so any other ideas or thoughts appreciated!