Anything that is added will be welcome.
My ideas are these:
Civilizations: The best thing would be to add those civs that already appeared in the campaigns: Gauls, Bretons and Huns (or Xiongnu). The first two should have a new architecture. The Huns should have Asian architecture, but with different houses (tents) and wonder.
Units: There is a number one priority, a Scout upgrade, available only to civs that do not have Chariots. To face in the late game when the gold runs out.
This could be the Light Cavalry. 70 HP (80 nobility) 5 attack and 10% more speed than other cavalry.
From now on, anything is welcome. Maybe a unit just for Celtic civs, (skirmisher?).
Maybe a battering ram, for more variety in the siege. A new ship, similar to the longboat (AOE2) so that the Celts do not have a fire ship or catapult.
And the always mentioned upgrades of the forgotten units, the Slinger and Camel Rider.