I'm really enjoying the game so far! I'm a fairly seasoned RTS player, with the bulk of my experience coming from playing Starcraft at a relatively high level.
One thing I'd like to see implemented in OTC is a more advanced and, preferably, customizable hotkey system. In Starcraft 2, players can fully customize their hotkey layout, which has given community members the opportunity to innovate and try to optimize the way players can use their keyboard hand. Here's a link for reference.
What I'd like to see in OTC is being able to have complete control of buying and selling your resources using only the keyboard. This would free up the mouse hand to focus on moving around the map, scouting, building management, etc. It would be a great way to increase the skill cap in the game without putting any burden on casual players.
There's a lot to manage in terms of buying and selling resources, so I'm not convinced it's viable to have it all done on the keyboard, but it would be good to test. Here's a mock layout I threw together really quickly. Each resource would be tied to a number or F key, and ctrl and shift keys would control the buying and selling actions.
Lets say water is assigned to the 1 key. Controlling it would go like so:
shift+1 - Sell 10 units of water.
ctrl+1 - Buy 10 units of water.
Going down the list, I think this could be the default assignments for each resource, with players being able to reassign them to whatever is most comfortable for them:
1. Water
2. Food
3. o2
4. Fuel
5. Alumium
6. Iron
f1 - Steel
f2 - Carbon
f3 - Chems
f4 - Silicon
f5 - Glass
f6 - Electronics.
Figuring out how to fit in the 100 unit buy/sell action is difficult. At first I thought about using the alt key as a modifier, but it can conflict with F keys. In before AltF4 joke. 
Maybe something like:
alt+shift+1 - Sell 100 units of water
alt+ctrl+1 - Buy 100 units of water.
I'm hoping I'm not completely missing a system like this already in the game, but since there's no game controls menu yet i'm not sure. I only learned about the ctrl+shift+down hotkey to sell all resources by watching the dev stream, which i know LOVE and use often 