What are the limits and prohibitions of "bed talk" between spouses, and what is the ruling on a husband asking his wife to describe other women or imagine certain things?

It is impermissible for a wife to describe a specific non-mahram woman to her husband, due to the Prophet's (peace be upon him) prohibition of that. This is out of fear that the husband might be enchanted by the description, which could lead to divorce or infatuation with the described woman. As for describing a non-specific woman, there may be no sin in it, but it is more prudent to close this door entirely.