Trebuchet
I think trebuchets would make a great addition to the game. With only four choices of offensive units, expanding the choices would increase the variability in every battle making every battle that much more interesting.
While I personally don't find much of a problem with cats, many people do. This unit would help counter the cats without the need of tons of cavs. If cats are the alpha omega of the battlefield, the best way to beat them is to create a better alpha omega.
The trick is to create them in such a way that you don't offset the pre-existing balance on the battlefield too much. Last thing you want is everyone using the same unit because all others shun in comparison. Here is how: Trebuchets ought to have unbelievable range such that well developed cavs still take 3-4 turns to reach them. Conversely, the attack power/crop consumption should also be diminished such that cavs are still the number one option against them. The losses between cavs and trebuchets should be comparable to cavs and cats. However, because the cats move so slow. Trebuchets can hit them enough times before they get in range to do significant damage. Lastly, trebuchets would be the only unit capable of hitting fully developed turrets outside of their range. This can put the defence at extreme disadvantage. As such trebuchets should have a very low multiplier ( <<1 ) against turrets such that taking a well fortified town with trebuchets may take several battles to complete (i.e a very long time, kind of like sieging a town takes a long time).
I think trebuchets could add a very fun aspect to the battlefield dynamics. Let me know what you think.