A port city built in ruins would probably have countless religions, actually. Gods of the sea, luck, community, wandering, knowledge, thieves, all sorts of gods would be venerated in such a region. That's to say nothing of any crumbling temples to gods dead or just forgotten that the ruins may hold. Perhaps the last temple of a god to be abandoned becomes that god's tomb when the god dies, could result in some rather interesting ruins.

What it could be is that each of our characters follow some different religion, and we've all been tasked to go and destroy / recover something from an old temple / tomb of a dead god. First we'd have to FIND the ancient temple, then get past any new residents or traps or anything, then get / destroy the object, then survive the escape back to civilization.