I'd like to have the option to extend walls over shallow water and perhaps some form of defenses that could cover deep water as well.
The shallow water walls could require a whole lot of regular stones as a base for building on, making them more work to build than just a regular land wall but otherwise being the same for defense values.
The deep water defense line could require something like iron, to form a chain that can be extended for small distances over (deep) water. As well as a pricy cornerpost-like structure to connect it to. A gate version could maybe be lowered to allow boats trough.
I think it'd allow for some creative base designs around rivers or various types of shore. Plus, it wouldn't change anything major about game mechanics as far as I'm aware.