I think the Campaign area is superb, including mare nostrom, but I think that the objectives are rather ridiculous in terms of needing to sink 2 aircraft carriers, battleships etc.
The other issue which is generally missing in the game and was in SH3 too, is the receipt of radio messages, which I realise are historic, but meaningless in pursuit of the mission objectives.
I would find the receipt of orders that one has to act on during the campaign to be the most meaningful and realistic addition that could be made (in addition to the superb work done so far).