Recently AoM Retold came out and it has a real good mechanic to assign newly created units to a control group when the rally point is set to a unit that already exists in a control group.
I think AoE4 would benefit greatly from a setup like, or similar that allows units to be automatically added to a control group. It would streamline the process of having reinforcements go where a player need them. While still maintaining skill to determine when to use reinforcements to defend a base for example.