First, laminate the following card sheets so that you don't have to do them for each class; OR print enough for all the classes you teach. Cut them apart and hand them one to each student as a guide of their knowledge.
Have students write a ONE PARAGRAPH (4-8 sentences, in ink) answer to the card they are given. Tell them that there is no right answer so you are only grading on the fact that they did explain an answer to the question posed to the best of their ability and why.
Over the years I have told my student that history is full of "What if…" questions and no one has the real answer, but I love them because it makes them 'THINK' which is just what I love having them do.
I have also included a set of four blank cards if you need them for your on "What If..." questions.