what reward? For me? I wouldn't care I rework my city so often I always have space available. I just need to redecorate now and again.
for players in general? Not a clue because what could you offer them that isn't already a function in the game? more coins or supplies no much point to those for the higher ages in general. FPs? Already have lots of ways to get those. attack and defense boosts? Again we have plenty. In the end you need the system first to determine what would be worth it for the player especially if you wanted to change it for all ages and not say the latest.
The real reward question is what's in it for Inno? For across all ages I can't see that be worth the time/effort. For a new age? Maybe but then you'd be in the spot where those players would have to give up much more beneficial buildings (think event buildings) and that won't go over well.
Is the only advantage you see to this system is making it more realistic (and of course a new challenge)? If so then you need to figure out what you want the system to be before you figure out what a player should get for mastering this new wrinkle since it needs to be worth it for the energy they'd have to put in to it. And once you've figured it out then what? it doesn't seem like it would do much more than give a slight change to how cities are designed and then be just another factor players need to account for when placing items in their city? That doesn't seem like much of a challenge after that one redesign.