The thing about sentries got me thinking about ways to indicate the good sentry spots, and what I came up with was putting a marker at each spot where a sentry was put up, and coloring it from red to green based on how long it stayed up. And maybe the luminosity of the spots could be based on representing the most common location at 100% and scaling down proportionally. So, for example, suppose there are 20 rounds played in all. During 12 of those rounds, a sentry is erected in the same exact spot. That spot would be a color between #FF0000 and #00FF00 representing the average length of time it stayed up between 0 seconds and, oh, let's be generous and say three minutes. Over on another part of the map, another spot was the home to 3 different sentries over the course of the test. So it would be a color somewhere between #3F00000 and #003F00.
Then maybe a second version of the same map could use colors between red (most) and blue (least) to represent the damage outputs of those respective sentries.