Alright, so obviously there is not going to be boarding and for very good reason, but that doesn't mean the game's combat shout be restricted to airships alone. Even though I have absolutely no experience in game development or concepts, I hope my ideas will at least be considered by someone.
When Adventure Mode is released, there is going to be trading between towns and factions at war. If the two towns are going to be fighting, it's unlikely that in reality the fight would be only in the air. I'm not talking about a Battlefield styled FPS game, but I do like the thought of a kind of siege mode in which one team is in airships attacking a town and the other team is in the town manning turrets on the ground. I have also seen fan art of tanks to be used in the game, though I don't know how much that would contribute to gameplay, if at all.
Another idea my friends and I had is larger airships. In the story of Guns of Icarus, there used to be massive airships, but that technology was lost, leaving everyone with the smaller, four person ships currently in the game. Well, I think it would be interesting to have the technology rediscovered and have ships with crews of ten and greater. This would also explain why there are light and medium guns but no heavy guns. The heavy guns would be on the larger ships. Also, with a teamwork based game, this would make the game much more teamwork oriented because everyone would have to pay attention to every subsystem on the ship.
Another concept that goes with the larger airships would be biplanes. A friend of mine mentioned that it would be interesting to have the ability to launch planes off of the larger ships for a more fast paced aerial combat. However, another friend pointed out that this would have the ability to destroy the entire premise of the game, teamwork. One player in an airplane might have the ability to lead the whole game, rendering the rest of the team inadequate. Which is why I also suggest the planes not be totally self-sufficient.
The first way to do this would be to require a pilot and a gunner. Instead of one player flying and shooting directly in front, there could be a player who flies the plane and another, or maybe two, players who man an ball turret underneath the plane and/or a tail mounted gun.
Another way to balance the biplanes, which I actually think is better than the previous option, is to have specialized planes. While there could be more advanced varieties than these, at a basic level there would be fighters and bombers. The fighters' guns would be very ineffective against the airships due to their size, but their mobility would make them great at shooting down other planes. Then there is the bomber, whose bombs would be very inaccurate. This would not be a problem against the airships because they are so large, but it would be extremely difficult to hit planes with them. This would make it necessary for a squadron of fighters to protect only one or two bombers. It would require more coordination because of the amount of people needed to simultaneously launch, as well as making sure the bombers don't get shot down. While there could be others, like heavy fighters for taking out groups of planes or interceptors specifically for taking out bombers, the basic concept would be the fighters and bombers.