In Simcity Buildit I have noticed that the NPC city has larger buildings than I have in my own. What allows you to upgrade to those? I have many times had no buildings left to upgrade because all were upgraded to their max. None of my buildings would offer upgrades until I built new building.
This makes me think there are some factors of the game that control what upgrade level a building can be at max. I have upgraded roads connected to buildings and still do not ever get upgrade options for them (and even when I run out of buildings viable to upgrade.
I should also note that my level is greater than the NPC player (he has changed level a couple of times after I passed him, but typically seems to take a few levels before that happens)