There really isn't a limit in what you can do, it's more a matter of time and your general ability. You can program pretty much everything you can do in Unity3d in Atavism, the difference being in how you send updates to the server for multiplayer since the client and server are separated. Could absolutely have airships I would think, not quite sure how that might work if that would be a giant mount or not, or how that would be tracked server side but seems possible.