Some suggestions:
1. Projectile trails
Right now projectiles are hard to see, especially when zoomed out. This is exacerbated by projectiles not having impact sounds, and the audio is out of sync so sometimes you have silent arrows or sounds that play midflight. Another easy way to fix this is by making the projectiles bigger (and I mean much bigger).
2. "I need [resource]" button for MP
Perhaps this can further be expanded into a 'Give [resource]' button prompt (100 default, 500 shift click) - so you don't have to open the diplomacy panel - and/or a chat wheel system, with "Yes", "No", "Help", "Attacking", etc.
3. Visible production/research progress bar below the capture point
IIRC this was in AoE 2 DE and I find this feature quite nice. Also useful to tell which buildings are idle thus maximizing APM count as well. In MP I see people queueing 5 women then forgetting about it.
4. Flags for unit destination
Unless you're queue building houses it's hard to tell where your units are going. We already have flags for garrison so I think we can use that.
Attached are what they could look like: