I’m not sure if I agree on throwing a mandatory prompt in the user’s face. That’s an extra click the user potentially needs to deal with every round. It’d be different if this were a game that didn’t require much menu navigation/clicking, but for A&A we already have:
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Purchase Units
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Dealing with the UI to perform Combat/Non-Combat Movement (clicking, hitting “undo”, etc.)
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Clicking through battles (one or more clicks for each round of combat (minimum one click to decide whether to press attack or retreat, possibly more for aesthetics)).
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Unit placement
One extra click doesn’t seem like much, but keep in mind it’s for a feature that only dedicated/competitive players seriously care about. Most people just want to build tanks as Germany/Bombers as the USA and mess around in a WW2 Setting.
I’d vote for just allowing you to right-click a territory during your NCM Phase in order to set custom OOL for said territory. If you there’s a need to modify the default OOL I’d just bury that in a User Profile or Settings Tab somewhere. That way it’s easy to track down for people who care, but not in the way of the experience.