I think a shopping centre would be cool. you could have a book shop with books taken out so dretches can get though to the next shop ect
A recently infested shopping centre (like transit is a recently infested train station) sounds very tremulous-like. Imagine your local shopping centre after hoards of tyrants had rushed through it and after thousands of bullets had been sprayed in every direction. Much better than the armies of salesdroids and screaming children you normally get

They also have large air duct networks, long corridors and many escape routes. The larger malls have storage areas full of tall shelves, beam cranes, catwalks and what not. Plus car parks and huge piles of industrial service plant with associated cable and pipework (hey, good trem maps need pipes, right? :wink: ).
It'd be a huge map, but since SST tremulous has been an MMOFPS...