Like being able to build trenches to stop the enemy flanking, clearing obstacles on the battlefield if a more open field suits you and setting various traps. Such tactics were used commonly in historical battles and they would give much needed tactical variety to TW battles. Most player tactics are a variation of engage + flank, if the AI built trenches and traps it would require more thought and a more cautious approach to win.
You could let an army prepare the battlefield if they are not overlapping with an enemy's zone of control. It would be done in a battlefield view without armies, sort of similar to city view in earlier TWs. Then if they stay in that place and get attacked their changes will appear on the battlemap. An army could get a certain amount of building points each turn and it wouldn't be worth it to build traps in forested areas because you would have to spend several turns using points to clear trees.
How about it?