I love your game, you put alot of work into it!
But my idea (as the title suggests) it so let talented community members have an option to make ships/planes/clothes (altough clothes are already being made).
Another option would be to look at the devs of the game planetside. They ask for faction based content, let the community create it (and even earn money from their creations) and sell the content ingame.
Playable planes:
Fighter planes (1 person) or bombers (2 or more persons)
This could add a whole new dimension to pvp. Keep the focus on airships (ofcourse) but planes with specific focusses like: bombers use fire bombs or fighters only use gatlings (or anty balloon or anti engine guns) to support (or escort) airships would we awesome. Especially in alliance or adventure mode.
Would also be cool in pvp but you should be able to do airship vs airship battles with no planes.
Looking at alliance mode with assault and defend i was thinking: maybe there could be a gamemode called battle. Implementing 2 factions that fight eachother (including ai). So a pvp with ai support.
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Tldr;
Player created ships
Playable planes
New gamemode called battle> pvp with ai support.