I, too, think this has some definite potential, but may not be necessary.
I presume we are talking about 1940, due to the SBR’s. I think adding mines into 1940 is an excellent suggestion, but prefer the 1914 method. Many merchantmen were sunk (in both wars) by mines (700 million tons of japanese shipping), and a few warships as well. Perhaps every ship supporting an amphibious assault from a mined sea zone must suffer them, and also any transport passing through the sea zone. This last may be OP, however.
As for coastal guns, they were installed in many locations along the coasts; sometimes artillery on atolls would target surface vessels. I suggest allowing artillery in any coastal territory to act as coastal guns. If an amphibious assault is launched on it, the artillery and bombarding warships have a preliminary duel, with the artillery each firing at two or less, and ships also with normal attack values. Warship hits can only be applied to these coastal artillery, and coastal gun hits are determined by attacker, but bombarding craft are first. If no offshore bombardment occurs, coastal guns may (for the first turn only, afterwards participate in combat as normal) fire against the attacking ground troops in the preliminary phase, with hits removed immediatly. if no coastal guns are present, offshore bombardment proceeds as usual. I do not think this is overpowered. More similar to your idea, I suppose you could also allow an operational naval base in the territory under attack to allow these guns to defend at 3 for the first round, but I am not sure about this.
And I do think it would add to the game; more interaction between land and naval units is always good! (ever had a huge armada floating around in the simpler editions, absolutely useless against the german tank stank growing in Eurasia!) :)