I guess they mostly do these gimmicks, because if you just toy around with different stats units and bonusses become too similar and turn out to be effectively the same for a large portion of the playerbase.
For example: Giving a unit higher attack speed vs. higher attack value
Of course we here, as the more enthusiast playerbase understand the practical difference and that the higher the armor of a target, you’d rather have attack value over sttack speed, … however in the grand scheme of things these minor nuances barely matter for the mayority of players. It matters that the unit with the bonus is noticeably superior to a generic counterpart.
So when you want to introduce new civilizations without gimmicks they wouldn’t feel significantly different to already existing civs.
So even if some of the gimmicks don’t have a much different effect then a simple stat tweak would have achieved, like “TheTowerDefender” suggests, I would assume the main aim is mostly to create the illusion of difference and variety.
At this point whatever can be done with stat changes has been done for the most part. More attack, more speed, more armor, more health, cost reduction, … everything has been done pretty much.
Argueably all combinations of unit lines like Archer+Infantry civ, Cavalry + Infantry Civ, Infantry + Siege Civ, … all has been done.
Now at this point, or argueably two or three expansions ago there is no more room to create civs that, even if its to some part an illusion, feel meaningfully different not only to a pro player, but also to a more casual player, that doesn’t crunch the numbers of when or where an increase in 20% more attack speed is different than + 3 attack, because on that level a win or loss is more dependant on the overall macro strategy than the marginally different outcome of a battle that occurs between these. You don’t really use Japanese Champions different than Aztec ones.
And that is why we now have Heroes, Aura effects, poison and burn damage, … because everything that could be done within the core AOE2 mechanics has been done already.
They have barely touched on the concept of “alternate stages” or “additional stages” for unit lines. There’s so much that can be done with that.
Perhaps there can be unique upgrades for buildings too, like a unique Keep add-on or replacement.
They should put Jar Jar Binks atop Bombard Towers, flailing his arms around in the air with his tongue hanging out, and rename them to Bombad Towers. Then I would understand their current state more 
Dont we already have this with multiple unit lines already?savar imp camel imp skirmisher legion winged hussar bohemian cannon.
If you take buildings only the tower line can do this which might be bit too much.
Well it you can always make it a bit stronger. to counteract pop efficiency. and after its dead HC will get out. Or sometimes you will not hide them in the wagon to maximize damage. :D.
Inside = good vs ranged. outside normal HC.
only feasible counter = cavalry and mangonels.
Why??? So you can arrive at the same thing but with extra steps??? This is stupid. It just penalises the person who wants to play Hussite Wagons by forcing them to go through this ritual every time they want to get some units out to fight.
Yes, but I just think it can be done more. Or rather, with a wider variety of unit lines.
The idea of either taking the time to aim accurately, or doing an inaccurate barrage fire, sounds like it could be an universal ability for ranged units. Take the time when it’s needed, or don’t when accuracy doesn’t matter and you’ll hit something anyway.
Well they would be totally different unit. As i see now people don’t like the unit its either broken or not used the way it was intended. (its not missile blocking thing for HC or archers couse it fires on its own and is great at that)… Its not a change its more a remake i suggested it would be closer to siege tower (transport units or just damage sponge) with ability for HC to shoot out.
There is a reason the siege tower is barely seen in any game and its because its clunky to use. The last thing we need is anothewr unit in the game that is clunky. Having a UU that requires another unit to be created and “merged” with it before it can dish out any damage is a terrible idea and would make that unit the WORST unit in the game.
And here I was thinking it was a good idea to make Siege Towers fire arrows if archer units are inside.