Honestly can't wait for news of the upcoming modular!
Lots of things I'd like to see, but I think an interesting way to guess what the future holds is to look at what has been done in the past:
Cafe, hotel, market, apartment, green grocer, apartment, fire station, department store, pet shop, apartment, townhouse, town hall, cinema, restaurant, apartment, pool hall, barber shop, detective's office, apartment, bank, laundromat, cafe, florist, bakery, music shop, photography studio, dentist, dance studio, apartment, diner, boxing gym, recording studio.
So I don't think it will be a food-based establishment, and I don't think it will be anything already done, as only apartments and cafes have been repeated, and the 2nd cafe was a tribute to the first. From the list of options posted above, that leaves museum, post office, gas station, library, gaming arcade, police station, hospital, school, DIY store, vet and pharmacy.
If they are deliberately sticking to the straight-straight-corner pattern, then gas station would be a big contender, as I think it could only really be done on a corner plate. It would obviously need at least one car, and I'm not sure if they would do a car in a set two years in a row, although the designer for the diner did say that they enjoy having a car in the build. I'm not sure about a hospital or school, as I don't think you'd normally see them in the main street of a town, but they could work. I think that a doctor's surgery would be more suited to the series than a full hospital though. I'd really like to see a post office, library, police station or museum, and think that they are very likely candidates. A gaming arcade is another that I'd like to see, but I feel like they wouldn't do it straight after that ideas project got rejected at the final stage. I think a vet would be too similar to the pet shop to work well, and I'm not sure about a pharmacy or DIY store, but they could potentially work.
Personally, I think out of all the options a post office is most likely, but honestly I'm expecting something completely unexpected, I think the series has taken a new turn with the diner, and am very excited to see what happens next!