If EA considered a second house to own, they'd need to double the amount of lots on offer, perhaps even triple the amount of lots on offer. I'm sure while the developers are being generous about the direction they wish to take, that is going way beyond generous though. i mean i'd love to have 3x as many lots in this game quite easily (obviously), to go from a little over 140 lots total to 420. however as it is, how are they going to be able to do such a thing with what they have at this point, unless they turn their small worlds into much much bigger worlds.
San Myshuno would be a perfect place to generate a lot of those additional lots, so many apartments and there are three i can think of off the top of my head within the arts district that are on the map but not used. there is another apartment building under the lounge on the top with the swimming pool not being used in the business district. the fashion district has a few other apartments right next to the Chic and Zenview apartments above a car parking structure that can be repurposed for use. the spice district has a good number of smaller apartment structures that could be used across the roads from culpepper and jasmine.
Then you got all these other places that have lots, Windenburg, Willow Creek, Oasis Springs... they need additional neighbourhoods or a more expanded area to use that can offer more lots. for example Arid ridge can be split up into multiple 30x20 lots and since the roads aren't being used anyway, perhaps they can be adjusted to allow for better access. or removed altogether, in addition there are house just outside the area that can be added into the world as well. the same with the neighbourhood that contains Afflusia Mansion and Yuma Heights, that area is grossly underused and there are several houses nearby that could be repourposed into a bigger map to be used at the very least.