I think 0 ad should release a beta before putting major new things like the following, which could be in an expansion later.
every unit should have a meter that measures their level of equipment and energy-- it would represent how many arrows or spears a range unit has or how tired a Melee unit has primarily. I'll call it prep for short. Every range attack uses a certain amount of prep, arrows use a little, spears use more. If a unit doesn't have enough prep it can't use its range attack and has a low Melee attack instead. Melee units use prep to do charge attacks. Prep rejuvenates slowly, or faster if units are near a supply wagon or armory. There could be special attacks such as fire arrows that use more prep.
terrain could do several things. Rocky or wooded areas would slow down cavalry some and things with wheels even more, and keep them from using charge attacks. Trees could give stealth or enhance pierce armor for units near them. Hills could enhance range attack as well as range and affect unit speed and charge bonuses too.