In Ashes you'd accomplish that through unit heat maps (which aren't in yet).
However, the biggest issue I have with zooming out so far has to do with unit speed. Players zoom out and then want the units to move faster. So then you make the units move faster and then players want to zoom out further and so on.
The Unit speed I believe is fine, you could add in faster land units though - but, I like the current speed of unit that we have. However, I think being able to zoom out, similar to Supcom or Sins, is really awesome and feels great. That is certaintly something that sets these games apart from the twitch fest like Starcraft. If you are worried that players will want faster units because they can zoom out further - then just add in more unit transportation options?
Maybe, like a Giant Land Crawler than can hold 20 Tier 1 units, or 10 Tier 2 units? It holds the units inside its hold and can move decently fast.
Or Air transports that can hold a smaller amount of units, and will be exposed to AA, but can move faster and over land obstacles.