+1 I love this idea.
Maybe have a 3 way option.
1) A very long one as per @Paeng
2) Like the Red Barn (Large Storage Barn in the Colonial Houses mod by @kid1293
) Where you have the main Building & then connect the extension to it as you need to expand the storage, treated as separate storage, but designed as the native extension (depending on coding, can you have the storage be coded to be directly affected by the trader, ie the trader exclusively uses that storage unless it is full??)
2b) Or, something like the 2 & 3 level houses, where you can overlay the buildings to make the dimensions different, longer, wider & taller as you want
3) Create a similar system like the Market Puzzle (again by @kid1293
, haha, a pattern is forming here, lol) and you can piece an aesthetically pleasing dock area with the different storage rooms (like the market, but not with the vendors) and you can make the layout as you want.
I would love to make a trading centre, which is what I'm trying to do now with the tiny & specialised markets.