How about, if you took someone's last city, you got a certain % of their investments? Or you got some kind of notice about how much money you had cost them? It'd just be nice to know.

Maybe if there was a way to differentiate between people who lost their only city, and people who lost all their cities, but there were more than one to begin with. Seems sort of harsh on newbs who only have 1 city.

well, they shouldn't be investing then. The idea would be to reward people who orchestrated a takedown of all of someone's cities before they could react.