I dont presume to suggest a graphical style i just like playing around with ideas and find pictures work better than words when talking GUI.
Upgrade all is a great idea when it comes to new features but you dont always want to do it as soon as it is researched. The upgrade all option has been suggested before and this idea ties in a little with another post i and others have made about keeping track of which slots have been upgraded ( a Slot Manager)
Im just a lazy typer… If you can drag and drop it im up for it … but the even the addition of buttons would make things easier.
Adding next/previous Axle (though this should probably read Schedule) buttons would be handy once you have several production lines going with multiple Front Axle slots on each.