I think Return of Rome should have some new units and upgrades.
Axemen line should get new upgrades, I would add for example Heavy Axeman and Falxman.
I think it would be best if axemen upgrades come from iron age unless people find that Short Swordsman should receive buff then it could come from bronze age with different stats.
Heavy Axeman:
75 HP, 9 attack, no armor and moves 5% faster than swordsmen line.
Falxman: (If barbarian civs would come to AoE1 I would reserve this unit for them, anyway only very few civs should have access to them)
85 HP, 12 attack, no armor, +5 vs hoplite line and moves 5% faster than swordsmen line.
Scout should also receive upgrade or upgrades. If they don’t want to upgrade them then they should receive +2 attack from bronze age like in aoe2 scout cavalry gets +2 in feudal age.
Bronze Age: Light Cavalry
80-85HP, 6 attack, no armor and +5% faster than other cavalry. Should also have res vs priest like chariots since some civs don’t get chariots.
Iron Age variant if needed: Lancer/Elite Light Cavalry etc…
100 HP, 8 attack, 1 pierce armor and +5% faster than other cavalry + conversion res.
Slingers need upgrades since less range than other range units makes it bad counter unit vs other range troops.
Bronze Age: Improved/Heavy Slinger
30 HP, 5-6 range, damage 3, pierce armor 2, is effected by melee armor upgrades and damage bonuses are same.
Iron Age: Elite Slinger
35-40 HP, 6-7 range, damage 4, pierce armor 3, is effected by melee armor upgrades and damage bonuses are same.
Camel Riders really need iron age upgrade, because they become pretty weak vs iron age cavalry units.
Iron Age: Heavy Camel Rider
140 HP, 8 attack, 1-2 armor, attack bonuses either same or +2 vs cav and +1 vs chariot/elephant compare to regular camel rider.
Heavy Cavalry should receive some health buff from 150 to 165.
I think that composite bowmen should receive Iron Age upgrade. Could be called Heavy/Elite Composite Bowman.
Heavy/Elite Composite Bowman:
50 HP, range 7-8 (should have higher range than mounted archers), damage 6, no armor.
Not sure but maybe Chariot Archers should have an Iron Age upgrade, could be called Heavy/Royal Chariot Archers.
85 HP, 7 range, 5 attack, no armor and has res vs priests like other chariots.
I think that AoE1 towers need damage buff here are my ideas:
Watch Tower from 3 to 4, Sentry Tower from 4 to 5, Guard Tower from 6 to 8 and Ballista Tower from 20 to 25. Or there should be similar tech like AoE2 arrow slits which improve tower damage.
Also I think rams should be introduced to Return of Rome. They should be available from bronze age. They called be called Battering/Light Ram, Armored Ram and Siege Ram. Since it would be totally new unit its hard to me give it even initial balance.
Bronze Age: Battering/Light Ram
150 HP, pierce armor 20 ( Ballista Tower should be capable to destory them so they should have 5 pierce armor less than basic ballista tower damage), damage should be better vs buildings compare to Stone Thrower.
Iron Age: Armored Ram
185-200 HP, pierce armor 35 (it would mean that ballista can deal 5 damage per shot), damage vs buildings should be better than Catapult.
Siege Ram:
250-300HP, pierce armor 40 (it would mean that helepolis can deal 5 damage per shot), damage vs buildings should be better than Heavy Catapult.
Anyway here are some unit ideas, which should or could be in the coming Return of Rome.
More upgrades to different units would make easier to make civs more different to each other.