First off I've had a blast with this game. Just creating the most absurd war machines is really great. I can't wait to see how this game grows, so props and keep up the good work.

As far as gameplay and a playable campaign goes I think my title post says it all. This game feels like a medieval Kerbal Space Program (and there is nothing wrong with that Kerbal is freaking great) so why not adopt their mission level ideas. You do missions for various lords, kings to get money. To earn new tech you do test missions for inventors where you are restricted with the technology you have to use. This would allow you to have similar levels but have the solution be very different. Just a thought I'm sure you have other stuff to focus on right now (more levels) but this is an easy way to add depth to your game. 

Quick idea, add smaller propellers 

Thanks for reading