More Balloons: Unless there is some reason why they shouldn't exist, it would be useful to have:
Barrage Balloon 100 m - To represent barrage balloons just being launched or which have been hauled down, as shown here:
https://en.wikipedia.org/wiki/File:N...upply_edit.jpg
Barrage Balloon 300m - representing very low-flying balloons, as was sometimes used to counter V-1 attacks.
Barrage Balloon 900m, 1500m & 1800m - to fit in at 300 m intervals between existing barrage balloon objects.
Of course, what would be really convenient is the ability to set the altitude of Barrage Balloons in the FMB, at any altitude between 0-2400m.
Observation balloons at higher altitudes - 300 m, 600 m, 900 m, 1200m, 1500 m, 1800m, 2400m.
Assuming observation balloon doctrine was the same during WW2 as it was during WW1, observation balloons usually operated between 1,000-1,500 m and could go higher (about as high as WW2 barrage balloons).
If you wanted to get fancy, a winch truck object could be added to the base of the barrage balloon or observation balloon objects, with the option of hauling the balloons down under certain circumstances. During WW1, the rate of descent was normally 2-3m, but up to 10 m/second in an emergency.