No matter, how much I circled with my mouse on my screen (and yes, I did that, more than once), my co-players weren't be able to see what I meant, even if their character was standing right next to mine.
Now, there are a lot of possibilities, I think. One could be some option I got to know from AoE - clicking on the map with the marker tool, and a blinking "X" would appear on the minimap. Another would be to use a (craftable) item, like a flare to place at a certain area, maybe a glowstick, or some colored smoke grenade. Yet another thing would be signs to write a message on, but there's no need to overcomplicate everything.
The advantage I see here would be that co-operation could be handled much more intuitively that way, people would know the locations to expand to, where to focus defenses, build factories, expand solar panel farm, ... and I think it would fit the setting quite well to have some (crafted) marking items at our disposal - but that's just my personal opinion
