Hi, thanks for playing!
1) I tried that, but it just didn't fit well with the design and in the end I decided it's not that important. Maybe I'll try again in the future.
2) I actually had something like this in mind when coming up with the game idea: You'll get a bonus for certain class combinations: E.g. 3 knights: -10% damage. However, something similar is now planned via the skill tree.
3) I'm playing around with the idea of just increasing damage by a factor the further you push into enemy territory (and vice versa).
Multiplayer is definitely something I have in the back of my mind, but it requires a lot of additional work and will probably not be implemented any time soon. One idea would be to be able to "upload" your team every X battles and see how you would perform against teams of other players in the same stage.