Several items here.
Add another special event (I would label it ‘Zeppelin L59’) which would allow the German player, at some cost in War Supply, to at least attempt to fly one airship resupply mission to German forces in Africa.
Reflecting the historical situation I would also allow Zeppelin raids against other target cities as well as against London. Attacks were made on cities in Britain, France, Belgium and Russia and should be allowed in the game. The affect on morale in the target country, at least from the initial raids, could be quite dramatic. In game terms there should at least be an award of victory points. Defending fighters should have to be deployed in the target provinces to attempt to prevent these raids from being successful. Also development of an effective fighter defense was not instantaneous as the game presents the issue.
German Long range bombers (and there were very few of them) should be allowed to raid targets in Britain, France and Russia similarly to Zeppelin raids with similar effects. British long range bombers also should be allowed to raid industrial and rail targets in Germany with the special event ‘Creation of the Independent Air Force’ being added to the game as well as allowing British bomber units to be built. In fact all of the major belligerents should have the ability to build and employ a limited number of strategic bomber units similarly, as aircraft technology advances. (The Russians could even be allowed to start the game in 1914 with a small compliment!)