From personal experience with A23 and reading all the forum posts I can, I'd like to summarize my own suggestions/features that could be of interest for the next release. I'd try to be as synthetic as possible, but of course we can develop this ideas further if there is interest/discussion.
Siege engines
Battering rams. Actually I find even sword cavs to have problems while chasing a retreating rams, lets not even talk about walking soldiers
Should have a lower base speed. Garrison units inside rams should increase the speed and even damage a bit
Pykes and spear soldiers should be able to damage them, to simulate the fact a ram is driven by other soldiers which will take damage from the enemy (but I think this would need to differentiate between archers and pykes/spears damage)
Bolts. By far the most efficient tool to kill enemies I have seen on A23
They need to be more vulnerable, specially when chased on retreat with again another base movement speed reduction
It could be nice to "garrison" units inside which could increase rate of fire and it's defenses. If this can be packed with additional animations showing the soldiers of that units, could be awesome
Catapults
Packing and unpacking speed should be raised, this would emphasize on the decision making when using them. Actually players can pack and unpack them soon enough to deny their opponents the possibility to retaliate when attack goes wrong
Should be easier to take down, with incremental resistance with garrisoned units
Siege towers. Actually have little to no use
Same as battering rams, should get lower base speed and increase it with garrisoned units but walking units should be still able to catch it
They should be able to capture at least certain targets (castles, cc, military colonies and towers, to not make them OP) to better define their role into the battlefield while being able to keep attacking - maybe at a slower rate when capturing
War elephants
They could have some extra armors in general but most important they must deal some damage around them, if possible only while walking to better simulate their use
Ranged infantry units
You may already know my opinion from other posts, IMO it's not about "ranged/slingers win games" but about choice. Actually there's dominant choice when composing an army: few frontline units (if any) and about 10 times those units as ranged. From this dominant choice, Bretons arise as the first choice civ: "one-size-fits-all" slingers from the beginning, faster eco, the security of being hard to be rushed and a fair possibility to be the first one to attack the enemy.
Archers
New feature: suppression. I will explain this later
Minimal range, same idea of towers. If a unit enters this minimal range, it cannot be shot and the archer need to fall back
Javelins
New feature: suppression. I will explain this later
Slingers
New feature: suppression. I will explain this later
Same health than other units maybe but definitely much less armor
Lower base damage, movement speed (they carry rocks), attack speed and remove blast damage but let them be improved by techs:
Clay ammunition: extends range and attack speed
Casted lead ammunition: improves damage
Also note Carthagians merc slingers should have a bit better stats than others, as Balearic Slingers were widely known in the ancient era for their capability. I guess this should also includes Iberians slingers, but I am not sure if that's historically accurate.
Features
Melee infantry block probability. Positional damage is still on developement, wouldn't be easier to add a block probability to incoming attacks meanwhile? I am probably wrong, but a similar process used from the game when deciding if an arrow hits or not, could be used to determine if a shielded unit blocks incoming damage or not
Suppresion. Archers and slingers should slow enemy units to better reflect their support role in the battlefield. Spears, but specially pykes, should also slow enemy cavs to increase their utility on the battlefield.
Terrain as strategic factor
River crossing
All units should move much slower when walking on water
Siege engines should not be able to cross rivers, but need to be transported instead
Trees
Slows cavs, as it's harder to maneuver
Protect from ranged attacks (maybe block feature related?)
Elevated places. I am not sure about this, but I am pretty sure the game already takes into account if a unit is on a higher place to extend their attack range. However, it should also be taken into account to extend their vision range A LOT more. This is specially true for towers and vision towers.
I am sure I left something behind, but probably it's enough to discuss for now. I hope you all enjoy the post and join a productive discussion.