They COULD sort of introduce the class system again, or something similar. A way to customize which units to utilize in the battlefield. I mean, many older units from the TW2 era would still be very useful, but they get discontinued. Like subterranean tech for Nod or Firestorm walls for GDI. They do have equivalents but some situations could call for a different arsenal.
Bring back some older units but upgrade them to suit the current age. You could have a pool of say 1000 points and each unit or special power would cost a set amount of points, this would call for many new tactics and sacrifices.
It just never feels you have the whole armys arsenal to your disposal. This might sound like C&C4 but it would be implemented in a much better way, without the stupid counter system and unlockables. Another way to go would be the possibility to customize the units themselves, from weapons, engines, attachments etc. Sort of like in the StarCraft 2 campaign.
The use of older things are common in real life too, for example, the M1 Abrams tank has been in service for over 30 years.
I'm just rambling :P