In Starcraft 2 you could use the mouse wheel when hovering over the “send resources panel” to very quickly add/remove resources to send in units of 100. Adding this to AoE4 would really help with sending resources to allies in team games. Currently, you have to spam click to send resources, and it can be quite painful when you want to send lots of resources in the heat of battle.
Another idea, this time from AoE2, is enabling shift-click to add larger units (e.g. 500 instead of 100).