A brainstorm of possibilities, of ways to take advantage of a passive stack:
Take an adjacent bonus, especially if you can expand there unseen. I.e. take a bonus one territory away or so from the stack which would normally be unsafe.
Tap the stack each turn from all adjacent territories. E.g. suppose you both deploy +5 and you have 5 surrounding territories and both start with 20 vs 20. You may prefer to maintain 20 vs 20, compared to 25 vs 25. Bigger stacks favor defence. But expect the opponent to wise up to this after a few turns.
Maneuver your stack and then when it's big enough and the time is right sprint away towards your opponent's main bonuses elsewhere.
Create a trap. Pretend to be taking a visible adjacent bonus and allow your stack to appear weak, while actually building a reserve behind and then send them forward and hope they crash into your surprise stack. This requires good timing.
There's probably more.