I see your point about digging fox holes. It would be unfeasible in a 90 minute scenario. To keep things realistic, I was proposing infantry digging hasty fighting positions. If you’re not shooting you should be digging and preparing a defensible position, I would presume. That level of detail is probably not worth it though, in retrospect.
Or, just having simpler fortifications besides a bunker would make scenarios more interesting in my opinion. 5 levels would be good. Hasty defense position, Fox Hole, 2 man fighting position, slit trench, bunker.
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Improved threat level would be interesting, also. For instance, have the option to not have a TC spend limited HEAT rounds on infantry in bunkers. Tanks Option Main Rounds: only engage hard targets.