We have thought for a while that it would be nice to have a timer to show the cumulative time your selected unit / building has been idle. This would be useful to beginners who would then be able to see how idle their TCs are in the early game, how idle their villagers are (after they select them all) and how idle their scout is. This UI feature could be toggled on or off, perhaps off by default, and could appear in the unit box, or in the top of the screen toolbar.
This goes someway to showing personalised interesting information about each unit, such as number of kills, time alive, buildings built, resources collected etc. This would not affect the game balance or how it is played, but would make spectating and casual playing a lot more interesting and fun. You can see the units that have been around for a long time and might try to save these units over others for instance, or micro the warriors who have the highest kill counts more.
Best luck continuing to innovate,