Well, the now-defunct idea would have been something like this:
Mines, both as a weapons of offensive area denial, and as a weapon of defense, were and are hugely important in naval warfare. HOI iterations, however, utterly fail to model the two reasons why naval attacks on coastal territories are never a straight run: coastal forts (while these are in the game, they dont, at least to my knowledge, have any lasting effect), and minefields. Now, they way I'd have implemented it would have been either by having minefields biuildable like infra, and destroyable via a new destroyer-only naval mission (clear minefield), or by being able to bulid a unit called "minefield", that functioned like gar for coastal sea provinces, and could be destroyed by extensive naval operations.
Now, in the "infra" idea, it would have worked like AA works in DH or AoD, whichever has AA batteries attack planes flying over, not just planes attacking the province. Crossing a minefield would incurs serious speed penalties for the enemy ships, along with strength and org losses, to model disarray and ship loss. Can be negated by having minesweeper-brigaded destroyers in the fleet.
In the "gar" model, minefields have high def value, but low attack, and their main objective is to slow down the enemy fleet by engaging it in a loooong battle.
But none of this shall come to pass.