With Arma 3, there has been plenty of expansion in Naval/Maritime/Underwater warfare, with the introduction of speedboats and divers. I propose that it be expanded once more with the introduction of small scale Naval Warships.
For relation, these ships are sized similarly to a yacht. Skjold-class patrol boats, Visby-class Corvettes, Hamina-class missile boats, the "Sea Shadow/Fighter" concepts and Houbei-class missile boats could be used as examples to illustrate the concept.
These ships would add greater depth and variety not only to Naval Warfare, but to gameplay as a whole. These ships could act as predators to the smaller "speedboat" classed ships available now, as well as engage each other in bigger skirmishes. These proposed ships could have enough firepower relative to the amphibious IFV's introduced in the Beta. As such, speedboat units will have something to fear other than air units. Air combat units will also have more challenging (but far from "impossible") naval targets to engage. Areas of maps previously overlooked (ex: the entire lower-right corner of the Stratis map) would have the potential to actually be used.
These ships could also behave as amazing "support" units, as well. Short range artillery could be deployed from them, behaving similarly to an MLRS, but with substantially less ammo to counterbalance their unique ability to traverse water. Other units could deploy from these ships as well; such as small UAVs/helicopters, or a conservative number of motorboats packed with fireteams or divers. Anti-ship mines could be deployed from these units as well, in a fashion similar to how mines are laid by soldiers.
I feel that these ships would be perfect for the terrain provided in Altis, as there are two giant bays which effectively cut into the island (save for the land bridge near the airport), along with plenty of tiny alcoves and inlets. Keeping the terrain in mind, Anti-ship missile batteries could be deployed as support along the coastline as well, giving even more depth to naval/invasion scenarios.
In summary, I think Arma 3 would benefit with the addition of small corvette/patrol class ships. One could engage in "true" (Ship v. Ship) naval warfare in this manner, and would give more depth to plenty of scenarios as they could behave as excellent "support" units, due to their ability to traverse locations on the map that many other units couldn't.
Besides, who wouldn't like the idea of seeing anti-ship missiles arcing over the Altis land bridge, while a full-on assault for the airport occurs? A "surprise" support boat in Marina bay during an invasion scenario for Stratis? The possibilities are endless.