My wife used to say "nothing" or that she was thinking about me when I asked her what she thought about when she came, especially in cases where I used a big dildo on her or my cock with a dildo / fingers. Eventually, after growing more open and with some insistence, she started admitting she was thinking about getting fucked by a huge cock or two cocks, multiple strangers, etc.
One time while ...... she actually revealed a rather detailed story (inspired no doubt by this viking show we watched) where a big, muscular viking busted in the door of her cottage, took out his huge cock, pinned her down and fucked her really hard.