Increase frequency of big calls based on expansion number of building. For example, something like +0.1% chance per building level (of station generating the call) to generate full building collapse. Or a reduction in chance of generating the lower level calls like smoking electrical equipment per building level. Alternatively, a system where the last 5 building levels, or the last expansion level gives one of those bonuses/reductions. This system would encourage more maxing of the buildings (which I'm sure isn't a problem in FD, but in PD and EMS expansions, at least in my experience, aren't as big of a deal) and would give more endgame stuff to do with credits and coins.
Also if that idea gets passed or approved, or whatever, some kind of counter on the profile page that shows how many of each type of station you've got maxed, at level 30 and/or level 20 to drive some more competition. It would be cool to have a stat, other than the credits leaderboard to look at.
*Also, loved the EMS update, especially medical school, pls more lol