Building on RJ's post, I would say that any really distinctive feature is undesirable. One of the larger issues for the stock set in post 1 is the shape of the building. Perhaps one building might be like that but, in a large group, the repeating patterns/shapes starts to look to obvious. When RJS released the Tudor set the distinctive features were made placeables and the building blocks themselves were just simple square and rectangular blocks. this allows the builder to create a randomness in the building structure for a group of buildings.
The issue then becomes the organic nature of buildings (the little nooks , crannies and non straight walls) that make a scene interesting, or more realistic, is lost.
Your building set looks really great. Perhaps if you create 3 different base buildings (we can scale in either direction so the three quickly become many more with the right use of textures), then a few shop front first floors, two or three door and window bays, a few lamps and signs (a sign post or two is important as the style of your building is very distinctive compared with stock items and the available sign supports in other cc sets). Perhaps even a selection of window sills?
The lit windows will create unwanted patterns if care is not taken. It's a very strong visual feature that will pull the eye. Therefore a window placeable or two will allow the builder to add to the stock model to create some lit/unlit windows that foil the pattern created by the repeated use of a base model.
With the based building blocks in place you can consider a few other items. Alley entrances are good, A two story building over an arched entrance perhaps. A building with a double door, arched and square to create inn and warehouse entrances. Some low walls with matching texture and architecture would be invaluable, A high level corridor or bridge between buildings would be superb to have in this set. Many set consider the street side but not what spans the street.
Lastly it is worth discussing the roof. That's a very architecturally specific roof. It's a good roof model but it is fixed and reduces the usefulness of the base. Consider it being a separate model ending at the flat area above the window. Then do a flat roof model with small wall round the top. Also a rotated stepped version so the roof slope could be from to back etc. With six models (three base, and three roof variants) you create 18 different base buildings frontages, assuming the base building block has a different front and back. I would also think about a two story and four story base box. That would give huge variation against time spent building.
Lastly but perhaps most importantly I have thoughts about the testutre and tint maps:
1. The buildings should be tintable. The tinting needs to be sensible (the stock tint sets are far to detained and often miss out the broad area that realy need to be tinted) Don't make only one section or feature tintable and not another, as the stock items often do. If I tint the brick on story three then the brick on story 2 should tint the same also.
2. Keep the texture map simple and then the moder can play with it to create new models easily. Keep the fine details off the texture map but try not to have the map pristine. Ivy on the bricks is bad because it creates repetition. The bricks should be generally worn without strong feature like broken bricks and cracked walls for the same reason. Creamily no painted sections of bricks or posters. these could be placeables if they are required to achieve your desired look. Many textures that look cool in isolation look crappy when used as they create a checker board effect that cannot be foiled easily.
I hope that's helpful thinking. I use a lot of models but cannot create them so my perspective is one of an area builder, I write with no thought for the time it will take you to do this. Only as a selfish modder, always hungry for new toys to play with 
PJ