Here are my ideas/suggestions:
1. Revisit the cafe concept. Cafe Corner was great but it lacked interior and many of us missed out on the first modular building. I would love to see LEGO revisit the cafe/restaurant idea, this time with interior - e.g. 1st floor "bar" area (strictly non-alcoholic of course ) plus kitchen, 2nd floor dinning area, 3rd floor/roof top "root beer" garden.
2. A car showroom/garage modular. This will have to be a full 32x32 building like Fire Brigade. 2/3 of first floor could be a car showroom and 1/3 could be devoted to a ramp or elevator that takes you up to 2nd floor garage/workshop where cars are being repaired.
3. Not so much a suggestion for type of building but a suggestion to add more variation to architectural styles. In a real city/town, you'll often see a mix of different style of building based on when they are constructed. All the modular buildings thus far have a late 19th/early 20th century look and feel, which are great. But I'm hoping maybe LEGO will add some more diverse style to the future modular buildings. How about a mid century modern bank building? Cape cod style post office? Art deco book store?