AoE1 is fundamentally imbalanced. Some units are just totally unbeatable when massed. There is nothing you can do to counter them. AoE2 does not have this problem, as even if 90 Paladins are flooding through your base, you can still make tons of Halberdiers to easily wipe them out.
AoE1 has no such counter system. Hoplites especially are totally unbeatable when massed. The wiki SAYS they’re countered by archers, but they have so much pierce armor by the late game that they take only 1 damage from most archers. This needs to change. There must be a counter to them.
Defenses in particular also need an upgrade. Towers and Town Centers should shoot extra arrows when villagers are inside (the latter can be unlocked by the tower technologies), and buildings need significantly more HP. AoE1 is extremely aggressive and not at all beginner-friendly. This needs to change.
Tool Age units and Camel Riders should also have further upgrades that deal more bonus damage, to improve the counter system. As it stands, there are no hard counters whatsoever, so any units flooding your base are unstoppable. Perhaps the Bowman and Improved Bowman lines can be combined, with the cost changed accordingly.
It would also be nice if archer and cavalry armor upgrades improved pierce armor in addition to melee armor. Infantry has the Shield technologies, so they don’t need that change. If that’s too much of a nerf to archers, the woodcutting upgrades can also improve attack as well as range.
Chariot units must also be nerfed. They should have a gold cost, so they can’t be endlessly spammed for days in the late game.
Changes that would be nice but aren’t needed are the additions of Fortresses, unique units, and unique technologies. Fortresses would be really useful defensively, combined with the increased building HP, even if there are no unique units or techs. It might be best to keep them out of the game to differentiate it from AoE2, but at the same time, it’s within AoE2, so appealing to the core fanbase is a smart move. If the AoE1 purists don’t like the changes, they can always go back to the CD or DE versions.
All suggestions except the last one are necessary changes to make the game fun and balanced, because it is way too frustrating, one-sided, and hyper-aggressive as it is right now.
Edit: Just another thought I had was the ability to garrison Artifacts in Temples to generate gold. This was an ability I gave the Israelites in my concept, but it really should be universal. Right now, they only work as scouts or a win condition in Standard conditions, so they’re basically useless in Conquest. Maps can be overhauled to always generate Artifacts instead of only doing it sometimes if this change is implemented.