Just an idea i want to ramble about...
Why not change the name of the TU system into something that sounds more realistic to the game and has potential to expand the system more appropriately? We know that each individual has a set amount of TU to perform actions and its also affected by weight but it could also have long-term properties such as: constantly overusing the same squad will gradually lower the combat effectiveness of each individual in ground combat (therefore a smaller limit of action points/TU). This of course can be restored by allowing the squad to rest up a bit. Or it can add more unpleasant features for an individual soldier, for example a soldier constantly using all his combat effectiveness may be more reluctant to follow orders, going so far as to out-right refuse.
This can be a feature that could open up to quite a lot of new possibilities like: soldier mental health and behaviour and more complicated short-term/long-term strategies.
However obviously i know this is a very hard feature to ever implement especially on a tight budget. Just think its quite a cool thought for a TU overhaul.