The upgrade of fortresses to bunkers is a very good idea.
About the SAM sites, I don't know... If you build them everywhere around your cities, the entire air combat should be compromised. Maybe with a limitation: let's say, no more than 2 SAMs around a city.
I think the tunnels construction is already included through the longer time the engineers need to build railroads in mountains.
But of course there could be a lot of new fun or usefull TI around our cities. For example, energy relating TIs: solar collectors, wind mills, etc; or national parks (increase tourism gold, reduce pollution), ski camps (tourism), ... Well, I'm too tired now, so I can't figure out other TIs, but there is a lot of place for good ideas here
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