A couple of things I've been thinking about:
Composite units - One, it can be very hard to double click moving units, it frustrates me all the time, especially during engagements with the enemy. Two, formations don't have to exist for heterogeneous selections of units. Both gameplay-wise and historically, I don't know of any reason why anybody would mix spearmen, archers and cavalry into a single fighting line. My idea is to make formations exclusive to a selection of units of the same type, and when you click a formation, you would automatically turn your selection into a composite unit. You would select the whole group with a single click and the individual units are no longer selectable. To break it, click "no formation".
Icons for control groups - I like the fact the control groups are visible and clickable. It's a great idea. It's hard to reach all the numbers with just one hand on the keyboard. I don't really like the way they look, though. Why not make them cuter and easier to use by giving them different icons? If the group is made of units of the same type, put the unit icon on the control group icon. For example, I like to put all of my Civic Centers into one control group, so the group would have the Civic Center icon. I use such groups for all of my buildings, so at least that should be simple enough. If my group is all soldiers, let the icon be some generic soldier-related thing like a helmet icon. Slightly more complicated: If they are all cavalry make the icon some generic horse. same for siege units. Some of the suitable icons are already present in the game.
Oddly enough, I wanted to suggest this exact thing
But for some reason I thought, "Wait, it would be really stupid if this already exists in the game, better check", and there it is Awesome.