From reading some of the Modding Summit threads it looked like some of the people in the know think it's possible to increase the building slots from 4 in small cities and 6 in large cities.

I believe it had something to do with the fact that there were now settlements (horde camps) with many more slots and they could be retrofitted into settlements.

I feel that with the addition of smaller sanitation buildings such as wells and even religious buildings that the game could do with a global increase of 2 slots across the board to all settlements. I understand the balance implications this has and have no idea what impact it would have on the AI and how they build but it'd be interesting to get an answer out of someone who may know!

Cheers