Hmm... I can see that too high tourism isn't good either. At a certain point high tourism should decrease itself. And it oculd also make the locals or cause a loss of culture as the city adapts to the strangers. High tourism can raise crime, it can be a bad thing for and when tourists block the public transport etc etc...
This is crucial. How about buidiings which bring in tourists having a population increase. But these extra pops would be useless (i.e. just taking up space but spending a few gold each). A class of specialist I guess..
At the moment the cruise dock adds 4 happiness, why? 99% percent of the time it is foreigners arriving by cruise and while they are there they are using your food and amenities. So the extra pop should be useless nad just take up space; in effect a happiness and health penaltyweighed against a gold bonus.
I see there being tourist "potential" determined by the tourist property and a tourist cap determined by a range of tourist buildings with varying effects above and beyond the pop "bonus". For example a 5 star hotel would add extra gold and a maybe great person points while a Hostel might add unhealthiness and crime.