Can I ask for a new category to be added or do I need to do a run in that category with proof before it will be made? For example, Prologue Hard Mode is currently not a category. Is this because of runner interest or is it just current status quo to not add more categories?

Generally not much of a difference from normal to hard, where as VH is instant death with basically any hit. However yes generally once some concept runs have been done and there is enough interest with the category and further expansion on it etc that is how they end up on the board.