1. Should we remove the Idea Generation close reason?
Yes. Much as I personally make use of it, and despite my generally good opinion of it, I also think it has its problems. For one, I don't think it was well defined when we added it, and was done in a little more of a hurry than perhaps it should have been at the time. Oh well, what's past is past.
However, it's currently not very well understood what we should use it for, and in what ways it's different from the primarily opinion-based close reason. There are differences, but they're subtle and not very obvious. At this stage, it would in my opinion be easier and in the long run better for the site to remove it and work out what we can do instead.
2. Should we add a What If close reason, and what should its wording be?
Again, yes. This is what we can do instead. We should add a close reason that specifically addresses what-if style questions. This will likely be similar to the primarily opinion-based close reason and to the Idea Generation reason, but will hopefully make it more obvious what's wrong.
Perhaps something like this would do the job:
What-If style question — questions that pose a hypothetical scenario and ask what the outcomes might be, without specifying details relevant to the worldbuilding problem, are both too broad and difficult to objectively answer, and are hence off-topic.