That reminds me of an idea i have had for a long time for use in an AoS. It was basically having an independent town off one of the lanes, or in some way that it could easily reach every lane. Basically, the town would start out neutral, but if you completed some requirement it would switch to your teams side and would start sending units out to help. I've tried a few times to fit it into maps I was making over the years (I started making maps in June 2004), but I could never find a way that made it balanced.
Every time it just seemed to let the winners just have more units, and the losing team could never actually get to the town to turn it back. One of the attempts was to have it be that you would have to stand on some sort of platform and it would turn, but that led to the town switching constantly as you could just run up and change it. Then I changed it to a time thing, that you would have to stand on it for thirty seconds, that didn't work either as the heroes would just get killed, or run. My final attempt, which was in a map that tried to combine RPG elements with AoS elements, involved completing a quest to have the town turn.
While my final idea was my favorite, it also didn't work as nobody would ever want to try to finish a quest instead of actually defending their bases. I never finished one AoS map, ever, and I mostly blame trying to fit this idea, and many others into them.