The process of planning may well help to stimulate new ideas, but there are a number of other techniques which can be used. Fresh insights into a problem often arise as a result of ideas which may seem silly, impractical or far-fetched at first sight, so at this stage you need to play about with a lot of different ideas - even if a particular idea is not very good, it can still suggest a different angle on a problem. This kind of mental flexibility is a very important part of thinking about an essay.
You'll then need to consider essay structure. This involves thinking about how your ideas relate to each other, and leaving out those which don't fit - a typical student might need to get rid of about a third of the ideas they've dreamt up at this stage. You might want to move from mindmapping to the more structured planning format of outlining.