It would be useful to have some kind of tool-tip when hovering over a map-marker, to indicate which player placed the marker, and what channel it was placed on.
Also, it would be good to have an indication of the current channel when you place the marker. There is a small customization window when you place a marker now. Shouldn't be a problem to add a small channel indicator there.
P.S. I didn't even know before that the current channel affects the visibility of the marker. So this should kind of also be a way to let people know about that.