Distance

Something that might be cool, and I would probably use, is a little ruler icon that I could click on one point and then another and get a result back in kilometers.

I look at port cities and am not sure if my cruisers can bombard them or just the battleships.

I look at where to build airports and find myself having to click on an airplane and trying to measure the oval and it comes down to a hair.

I see units with different speeds and have to pretend an attack to see the time it takes to get there, then stop them.

Not sure if it would even be used by more experience players.